Bergvall worked in Ange's system as that one or even required ball-carrying 8s in the midfield three, so there was plenty of room for Bergvall, Sarr and Gray, especially with additional European games
Frank's system, on the other hand, needed the CMs to do a lot of quick passes...and since we didn't have those, he overcompensated by focusing solely on the wings (or to be more accurate, the right flank) and he moved both Bergvall and Sarr up the pitch
Which leads to the real problem: as revealed by Ali Gold back in February, Frank would talk with the leadership group daily but the rest of the squad would regularly go days without any one-on-one time with Frank, which in Bergvall's case was especially problematic because if he wanted to retool him as a 10 that requires regular feedback and Frank wasn't offering it
...while in comparison, Sarr had less issue with his new role given he was a high-pressing 8 under Ange, so he had less issue adapting to his role as a 10 in Frank's system as he was playing a broadly similar role