Been using synapse 4 with no issues up until that point.
Affected devices - Tartarus V2 Pro, Basilisk V3, Deathadder V2, Blackwidow V4. Most (not all) keys just reverted to default binds. Chroma is working fine. My wireless Blackadder X2 Hyperspped keybinds, however, curiously work just fine.
Uninstalled synapse twice, created brand new profiles with new keymaps - nothing.
What a load of absolute dogshit.