BUG: Non deterministic level order in MultiIndex with join
- [x] closes #36910 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This fixes the issue, if the number should be deterministic. If not, we should add a note in the docs
thanks @phofl nice!