I noticed this when I exported audio and saw the screen jumped around a bit and the exported mix doesn't sounded right.  Then I saw that it defaulted (not consistent though) to a Mix Scene, and I've always used 'entire mix'.  Strange behavior, is there a way to set the default or have it sticks to whatever you have set to prior (the good old way)?

If you use a single bus to output and listen to your mix i.e. master bus use that as the export bus. I have not used entire mix for export since Pro Audio.

Did a little more digging and see that if you have any mix scene saved, the option to export the mix scene will appear.  After rebooting the behavior is consistent, i.e., it will remember your last selection.