REGR: fillna not filling NaNs after pivot without explicitly listing pivot values on 1.1.x
- [ ] xref #36495

yet another alternative to fix directly on 1.1.x pending further discussion in #34407 on how to resolve the issues on master.

cc @jorisvandenbossche @jbrockmendel 

This won't close issue until fix on master. could copy test to master with xfail (and would need to sync release notes if we merge this direct to 1.1.x)
travis environment setup failure unrelated Hello @simonjayhawkins! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:





There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-09-26 15:05:56 UTC Fine for me to merge this now in 1.1.x, and have another version applied to master with the test xfailed I'll try to find the underlying problem so this is unnecessary.  In the interim, this is fine by me. may need more discussion.

I can't block by requesting changes on my own PR, moving to 1.1.4 so release note needs to be moved. closing for now instead. Will re-open after 1.1.3 release.

I've started the final release readiness checks. merging now would require going back to square one. Why not include this in the release? This PR is ready it seems (small fix, tests pass) merging directly to 1.1.x can be a bit messy. If there's a better way https://github.com/pandas-dev/pandas/pull/36668#issuecomment-699524035 we can do that for 1.1.4

If this issue still persists when we are ready for 1.1.4 release we could merge this instead? Sounds good!