Im having the same issue. The export has the bnlack frames in, but sometimes closing and reopenning premeier removes them, and then they are also removed in the export.
The footage im using is recorded in a professional studio on a Black Magic URSA, by ex-BBC staffers. There isnt an issue in the footage (plays fine, black frames arent in there in the footage, arent there played back in VLC etc).
I am using the Premiere Pro Beta 22.214.171.124 (but has occured in previous versions - including stable releases)
I am editing on PC, Intel i9-10900K CPU @ 3.70GHz 3.70 GHz, 64GB RAM, Radeon RX 5700 graphics card, Windows 10 Pro.