MM_FILTER_RULE_TTY_WITH_NET overrides ID_MM_DEVICE_IGNORE
From the MM mailing list.
With the following udev rules:
/etc/udev/rules.d/78-custom-mm-blacklist-internal-modem.rules :
ACTION!="add|change|move", GOTO="custom_mm_blacklist_internal_modem_end"
ATTRS{idVendor}=="[redacted1]" ATTRS{idProduct}=="[redacted2]", ENV{ID_MM_DEVICE_IGNORE}="1"
ATTRS{idVendor}=="[redacted1]" ATTRS{idProduct}=="[redacted3]", ENV{ID_MM_DEVICE_IGNORE}="1"
LABEL="custom_mm_blacklist_internal_modem_end"
MM is still allowlisting ttyACM0
ModemManager[3751]: <debug> [filter] (tty/ttyACM0): port filtered: device is blocklisted
ModemManager[3751]: <debug> [filter] (net/enxd[redacted4]) port allowed: net device
ModemManager[3751]: <debug> [filter] (net/enxd[redacted4]) port allowed: net device
ModemManager[3751]: <debug> [filter] (tty/ttyACM0): port allowed: device also exports a net interface