iPad: no fullscreen on external display with stage manager

Hi, i’m trying to connect my iPad (M1) to my TV (mirroring deactivated in settings) and use Wahoo SYSTM in fullscreen. When i click on the 3 dots at the top of the window the menu shows enter fullscreen mode, choosing this actually shrinks the window instead of enlarging it to fullscreen. With other apps this feature works as to be expected and shows the corresponding app in fullscreen. Is this a bug in the iOS app?