BUG: Fix droped result column in groupby with as_index False
- [x] closes #32240
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

The aggregate function *lost* the results of the aggregate operation, if the ```as_index=False``` flag was set and one of the result columns is relabeled. The index was reset at first, adding columns to the DataFrame. The relabeling operation only took the columns before resetting the index as input. So the weird error occurred. I changed the order of the index reset and the relabeling. This fixes the issue.