Sorry about the really late update here -
I did eventually figure out a solution by ‘jittering’ the FOV of the viewport camera slightly which appears to force the viewport to update.
That’s really interesting. I’ll check that I can consistently repro it, and if I don’t find a definitive solution or a workaround that isn’t weird and hacky I’ll look into making it a bug report.
I did actually try something similar to this previously to no avail - it appears the viewport is only forced to update when either it or its camera have their properties changed and so the FOV was the only workaround that worked for me. Thank you for offering help regardless!