Incoming network (ping/sshd) inaccessible after LightDM starts in Debian 12 (bookworm) on a 2010 iMac?

(Previously posted on https://superuser.com/ without much uptake ...) Debian 12 (bookworm) running on a 2010 iMac. After the machine boots but **before the window manager (LightDM) starts** I am able to ping and SSH into the machine from a remote machine. Once LightDM starts I can't ping or ssh from remote any more (and a running ssh session hangs). (I think this was also happening with GDM before I switched window managers, but not 100% sure and don't feel like switching back to test unless someone thinks it would be useful). service sshd status says sshd is still up and running, and I'm able to ping and SSH from the machine into itself (from a terminal window or from an alternative console via Alt+Ctrl+F1). Does LightDM do something to networking/firewall settings when it starts? I haven't been able to find anything useful in dmesg (but admittedly I don't know what I'm looking for). The machine has a static IP address, and as far as I can tell from ifconfig output the machine still has an interface open at that IP address (enp2s0: flags=4163 ... inet ) Happy to provide further diagnostics on request.