Sarah, Nick,
When I have the flash on the camera (in the hotshoe - I had mentioned using also the remote trigger but now I mean flash directly on the camera) but the flash is switched off (I mean by the physical switch on the flash, not in the menu of the camera), I see “Sync Terminal” in the Flash mode settings in the camera menu instead of the normal TTL-M-Multi-Off values cycled by by the rotary dial. When I switch the flash on, “Sync Terminal” disappears and everything works normally. This is not much help, I am afraid, but just to make it clear what I meant.
I could imagine drained batteries in the flash could cause a problem but I am sure Sarah tried that one. I once had funny behaviour of my older camera, an X-T2, with my Godox flash, when the battery in the camera was at a low charge level (and it was a third-party battery). After I put another one in everything worked fine again.