BUG: df[col] = arr should not overwrite data in df[col]
- [x] closes #33457
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

cc @jorisvandenbossche this still fails 7 tests locally and there's one more (commented in-line) test that looks fishy.  Extra eyeballs would be welcome.

xref  #35271, #35266