Check in Options->Controls->Devices. It sounds like something is constantly holding down a camera button. The game listens to all the detected devices simultaneously, so if there's anything in the Devices dropdown that seems unfamiliar or you aren't using, then select that device and set the Scheme to Disabled for that device.

Maybe that was the case after all. I restarted my steam deck (for external reasons) and no longer face that issue, so I'm not sure what was wrong!

Funnily enough, I read a very similar comment of yours that was a few years old but couldn't figure out if that was the issue; glad it fixed itself! I can say for sure though that what I saw then was what I see now, namely: "keyboard" (don't have any nearby bluetooth keyboards (that I'm aware of), oddly), and "microsoft x-box 360 pad zero" (despite not having an xbox 360 controller that could have any juice in it (the last time I used one was years ago)) as the devices, the latter of which had/has the selected scheme as steam deck

I appreciate the very rapid reply despite the game being an old one :)

You might be having some slight or occasional stick drift then - could be worth checking in Steam->Settings->Controller->Test Controller Inputs, and if so then you may need to set up a deadzone in the controller calibration.