Xcode 12 not parsing headers correctly (but only on Big Sur)

I have a big project file, objc/xib based. The file's owner class header is not being parsed correctly, the context menu for the "File's owner" shows warning signs for all existing outlets and actions.

What's confusing is this only happens when running Xcode 12 on Big Sur. On Catalina everything works fine. Makes it very tiring to test things cross OS (reboot to tweak the xib, reboot to test).

I'd gladly submit a FB about this, but first thing they'll ask is how to reproduce it. Smaller XIBs don't exhibit this.

Is there any log or sanitization tool I could try to narrow it down?