systemd version the issue has been seen with
237-3ubuntu10.11

Used distribution
Ubuntu 18.04

Expected behaviour you didn't see
service systemd-networkd restart successfully restarting the network

Unexpected behaviour you saw
Could not drop route: Connection timed out / Could not join netdev: Connection timed out

Steps to reproduce the problem
Load a full BGP table / have bird running synching multiple full tables and execute systemctl restart systemd-networkd with an unrelated / no change.

We consistently saw systemd-networkd restart again and again, never successfully bringing up the network after the initial fault.
Stopping both bird and bird6, then restarting seems to work but is highly undesirable as this always introduces huge routing changes internally and externally.