REGR: Different results from DataFrame.apply and str accessor
- [ ] closes #38979
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry


@jbrockmendel despite my comment https://github.com/pandas-dev/pandas/issues/38979#issuecomment-760470446, I had no joy trying to locate the caching issue.

thanks @simonjayhawkins  @meeseeksdev backport 1.2.x