dhclient issuing DHCPDECLINE 60 seconds after "successful" config
Somewhere between NM 1.36 and 1.39.7-2 (dhcp-client 4.3.6-47 hasn't changed versions), exactly 60 seconds after dhclient receives an IP and configures the system, it issues a DHCPDECLINE and grabs a new IP. This is bad, because it breaks NFS mounts, etc. for a minute or two until it can recover.
This is reproducible after boot; just kill the dhclient PID and it'll grab the old IP and reconfigure the system. Then, after 60 seconds, it will issue a DHCPDECLINE and request (and get) a new IP. The .lease file just grows with new IPs every time this happens.
The DHCP server isn't out of IPs/leases, there's no ARP or IP conflicts in the network. Booting back into NM 1.36 fixes the problem.
I've increased NM logging verbosity as much as I can:
07:40:12 localhost.localdomain NetworkManager[3994]: <debug> bus-manager: (dhcp) closed connection 1111111111111111 on private socket
07:40:12 localhost.localdomain dhclient[4321]: DHCPREQUEST on eno1 to 255.255.255.255 port 67 interval 6 (xid=0xaaaaaaaa)
07:40:12 localhost.localdomain dhclient[4321]: DHCPACK from eno1 to 255.255.255.255 port 67 interval 6 (xid=0xaaaaaaaa)
07:40:12 localhost.localdomain NetworkManager[3994]: <debug> bus-manager: (dhcp) accepted connection 2222222222222222 on private socket
07:40:12 localhost.localdomain NetworkManager[3994]: <debug> dhcp4 (eno1): DHCP event (reason: 'REBOOT')
07:40:12 localhost.localdomain NetworkManager[3994]: <info> dhcp4 (eno1): -- acquired IP info here as normal --
07:40:12 localhost.localdomain NetworkManager[3994]: <info> manager: startup complete
07:40:42 localhost.localdomain NetworkManager[3994]: <debug> ip6 timeout/fails
07:41:07 localhost.localdomain NetworkManager[3994]: <trace> solicit: router solicitation sent
07:41:07 localhost.localdomain NetworkManager[3994]: <trace> solicit: schedule sending next solicitation in 57.435 seconds
07:41:12 localhost.localdomain NetworkManager[3994]: <debug> bus-manager: (dhcp) closed connection 2222222222222222 on private socket
07:41:12 localhost.localdomain dhclient[4321]: DHCPDECLINE on eno1 to 255.255.255.255 port 67 (xid=0xaaaaaaaa)
07:41:15 localhost.localdomain dhclient[4321]: DHCPDISCOVER on eno1 to 255.255.255.255 port 67 interval 6 (xid=0xbbbbbbbb)
07:41:16 localhost.localdomain NetworkManager[3994]: <debug> bus-manager: (dhcp) accepted connection 3333333333333333 on private socket
07:41:16 localhost.localdomain NetworkManager[3994]: <debug> dhcp4 (eno1): DHCP event (reason: 'BOUND')
07:41:16 localhost.localdomain NetworkManager[3994]: <info> dhcp4 (eno1): -- newly acquired IP info here --