See if it were that problem, then there wouldn't be a problem with the PM's in the first place because it wouldn't read the postbit_legacy which I made edits to.
Well it's obviously displaying postbit_legacy just from looking at it. But the way the postbit/legacy switching works is weird, and like I said, when you turn on template names in html comments it says "postbit_legacy" when viewing the forum, but "postbit" when viewing a pm, so I was guessing that the preregistering was also getting confused. It could also be what Lynne said except that I'm pretty sure postbit_display_start is called before the template is rendered.