BUG: groupby.value_counts subset arg not working
- [x] closes #46383
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests)
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

It works but I'm not sure it is the right approach. Alternatively I could add an if-else in l.1712 to check for subset and create `keys` with two separate list comprehensions and leave the rest as it was.
One check is failing but can't reproduce locally. Not sure what to do.