We should of course default to feature detection whenever possible, but non-standard behavior like this in certain browsers is exactly why feature detection alone is never going to cover 100% of current use cases for UA detection.

To add to OP's point, this thing in Safari also comes to mind as an example of something that isn't easy to detect and address outside of UA detection: https://github.com/vitr/safari-cookie-in-iframe

Deprecating UA Strings and moving towards UA Client Hints seems like a move in the right direction though.