PWM Comparator types for Class D
It turns out this wasn't a design/electronics issue, but something within Multisim.

I'd been working on several different designs on the same design sheet and had convergence problems previously, running the Convergence Fixer a few times, so it had probably changed some settings that are beyond me. I won't say it was a Multisim fault as for all I know the program was working as intended.

Lesson learnt: If breaking the problem in to pieces (as a concerned citizen advised) make sure to keep as many constants as possible, in this case, running the simplified comparator circuit on the same design sheet so as to use the same parameters.