If you select entities on the add missing entities tab, then navigate back before that tab, it will forget your selections.
Example way of reproducing it:
- Go to http://test.musicbrainz.org/release/add
- Enter some information, including an artist name, but don't select an existing artist.
- Click the tracklist tab and enter something there.
- Click the add missing entities tab and select an option for the artist you didn't select
- Click the edit note tab, realise you forgot something
- Click the release information tab, add some more information
- Click the edit note tab again - you're taken to the add missing entities tab again.