The access-denied issue appears to be a regression from a recent change, it should now be fixed with this commit:
https://github.com/discourse/discourse-reactions/commit/69b1fd45a43333508a37684d11924c3fadc8d2dd
Logged in users should be able to see the reactions given by users from the Activity tab on the user profile again.
The other issue of repeated requests and page jumpiness still needs further investigating.