Myth or not?

I heard that you CAN’T switch between the two sections A and B during the ppr. More specifically, that once you move onto Section B (Assume you’ve done half of Section A) then you CAN’T go back to Section A or something.

I took the ACCA specimen paper (online one) and there was nothing of the sort there but… can someone please confirm this? I can’t imagine something more out of a nightmare than sitting for the ppr and finding this out the hard way.