I am having the same issue.  I recently bought a Samsung Galaxy Tab S10 Ultra.  Are you maybe playing on one of the Ultra tablets?  I think it may be related to the Ultra's wide screen.  I tried all the suggestions I found, like setting the screen to not full..  But nothing seems to fix it.   It's a pain to do this, but a work around is make the plants you want to select for Arena or Adventure  favorites so they appear at the top of the plant selection.    So you can select them without  having to scroll down.  I found comments made for this same issue from 4 years ago on this forum.  Following is a link.  The last comment on this link is mine.

https://forums.ea.com/discussions/plants-vs-zombies-2-en/re-the-plant-picking-part-from-a-level-is-lower-on-the-screen-and-i-can%E2%80%99t-scroll/10264655

I found a fix for this issue first on a Google search and then I also found it on this forum.   The fix is to go into Settings > Display > Camera Cutout.  Select Hide Camera Cutout for Plants vs Zombies 2.  It works on my Samsung Galaxy Tab S10 Ultra.