Auto Sync in Survey mode doesn't sync changes made to the stars under the thumbnail, though it does sync changes made to the pick and rejected flags under the thumbnail, to the stars in the toolbar, and to the stars with keyboard shortcuts.
To reproduce:
1. Select four photos and go into Survey mode.
2. Do Metadata > Enable Auto Sync.
3. Click and change the stars under one of the thumbnails -- notice that the stars do not change for any of the other thumbnails.
4. Click the pick and reject flags under one thumbnail, and notice that the other thumbnails also get the flags.
5. Click and change the stars in the toolbar (not under the thumbnails) and notice that the stars change for all the thumbnails.
6. Use keyboard shortcuts (e.g. 1 ... 5) to change the stars and notice that they change for all the thumbnails.
Tested in LR 10.0 / Mac OS 10.15.5.