Option `--test-multiplex-requested` - failed to create net link for device: failed to add link for device: Could not allocate link: Failed to add l>

Hi,

With MM configured with --test-multiplex-requested, MM fails to establish mobile data connection at boot time with this error:

mars 21 11:44:51 pinibrem5 ModemManager[751]: <warn>  [modem0/bearer1] connection attempt #1 failed: failed to create net link for device: failed to add link for device: Could not allocate link: Failed to add l>

mmcli reports 3 bearers instead of one:

  ----------------------------------
  General  |                   path: /org/freedesktop/ModemManager1/Modem/0
           |              device id: 42e44d34c8c7edad0b01488b6c3886a3e421b0bd
  ----------------------------------
  Hardware |           manufacturer: QUALCOMM INCORPORATED
           |                  model: 0
           |      firmware revision: MPSS.JO.2.0.2.c1.1-00032-9607_GENNS_PACK-1  1  [Feb 25 2019 01:00:00]
           |         carrier config: default
           |           h/w revision: 10000
           |              supported: gsm-umts, lte
           |                current: gsm-umts, lte
           |           equipment id: 863974040062624
  ----------------------------------
  System   |                 device: /sys/devices/platform/soc@0/38200000.usb/xhci-hcd.4.auto/usb1/1-1/1-1.2
           |                drivers: option, qmi_wwan
           |                 plugin: broadmobi
           |           primary port: cdc-wdm0
           |                  ports: cdc-wdm0 (qmi), ttyUSB0 (qcdm), ttyUSB1 (at), ttyUSB2 (at),
           |                         ttyUSB3 (at), wwan0 (net)
  ----------------------------------
  Status   |                   lock: sim-pin2
           |         unlock retries: sim-pin (3), sim-puk (10), sim-pin2 (3), sim-puk2 (10)
           |                  state: registered
           |            power state: on
           |            access tech: lte
           |         signal quality: 52% (recent)
  ----------------------------------
  Modes    |              supported: allowed: 2g; preferred: none
           |                         allowed: 3g; preferred: none
           |                         allowed: 4g; preferred: none
           |                         allowed: 2g, 3g; preferred: 3g
           |                         allowed: 2g, 3g; preferred: 2g
           |                         allowed: 2g, 4g; preferred: 4g
           |                         allowed: 2g, 4g; preferred: 2g
           |                         allowed: 3g, 4g; preferred: 4g
           |                         allowed: 3g, 4g; preferred: 3g
           |                         allowed: 2g, 3g, 4g; preferred: 4g
           |                         allowed: 2g, 3g, 4g; preferred: 3g
           |                         allowed: 2g, 3g, 4g; preferred: 2g
           |                current: allowed: 2g, 3g, 4g; preferred: 4g
  ----------------------------------
  Bands    |              supported: egsm, dcs, pcs, g850, utran-1, utran-5, utran-8, utran-2,
           |                         eutran-1, eutran-2, eutran-3, eutran-5, eutran-7, eutran-8,
           |                         eutran-20, eutran-38, eutran-40, eutran-41
           |                current: egsm, dcs, pcs, g850, utran-1, utran-5, utran-8, utran-2,
           |                         eutran-1, eutran-2, eutran-3, eutran-5, eutran-7, eutran-8,
           |                         eutran-20, eutran-38, eutran-40, eutran-41
  ----------------------------------
  IP       |              supported: ipv4, ipv6, ipv4v6
  ----------------------------------
  3GPP     |                   imei: 863974040062624
           |          enabled locks: fixed-dialing
           |            operator id: 20820
           |          operator name: -Cdiscount-
           |           registration: home
  ----------------------------------
  3GPP EPS |   ue mode of operation: csps-1
           |    initial bearer path: /org/freedesktop/ModemManager1/Bearer/0
           | initial bearer ip type: ipv4v6
  ----------------------------------
  SIM      |       primary sim path: /org/freedesktop/ModemManager1/SIM/0
  ----------------------------------
  Bearer   |                  paths: /org/freedesktop/ModemManager1/Bearer/3
           |                         /org/freedesktop/ModemManager1/Bearer/2
           |                         /org/freedesktop/ModemManager1/Bearer/1

All 3 bearers have the same status:

  --------------------------------------
  General    |                     path: /org/freedesktop/ModemManager1/Bearer/1
             |                     type: default
  --------------------------------------
  Status     |                connected: no
             |    connection error name: org.freedesktop.ModemManager1.Error.MobileEquipment.Unknown
             | connection error message: failed to create net link for device: failed to add link for device: Could not allocate link: Failed to add link with mux id 1: Netlink message with transaction 10 failed
             |                suspended: no
             |              multiplexed: no
             |               ip timeout: 20
  --------------------------------------
  Properties |                      apn: net26
             |                  roaming: forbidden
             |                  ip type: ipv4v6
             |             allowed-auth: none, pap, chap, mschap, mschapv2, eap
  --------------------------------------
  Statistics |                 attempts: 4
             |                 attempts: 4

These steps restore a working configuration:

1. disable mobile data in g-c-c
2. restart ModemManager via systemctl restart ModemManager.service
3. enable mobile data in g-c-c

Other things tested:

1. disable mobile data
2. reboot
3. enable mobile data

-> fails the same way

1. disable mobile data
2. reboot
3. restart MM service
4. enable mobile data

-> fails the same way

When not using --test-multiplex-requested there is no issue: the mobile data connection is correct with only one bearer registered.