BUG: Setting DTI/TDI freq affecting other indexes viewing the same data
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

The issue this addresses is related to #31218.
This is going to take more work, mothballing. this is good thanks. pls push a whatsnew note at some point if you would as this is a slight behavior change.