Quick update:
I dug further, and found out that the reason that the camera issue was fixed was *because* I remapped the camera rotation keys to the extra mouse buttons to work around the issue (go figure)...
Turns out that the previous game profile for the mouse buttons interfered with the default mapping and as soon as I started a new profile, it freed the associations to work properly again. Tested & confirmed by toggling the Razer mouse profiles and fully repeatable.
TL:DR - self-inflicted, but fixable!