We had created a number of topics before I updated the list of reaction icons, in Admin → Settings → Discourse Reactions.
Topics created after I changed Discourse Reactions do have the new list, but older topics now only have the single, default heart reaction, no list at all.
I’d like all topics to have the full range of reactions.
Once you enable Reactions it should work for all posts no matter how old, with no need for a background job. Though I think a page refresh can help.
If you’ve already cast a Like on a post then that will be locked in after 10 minutes and won’t be able to be removed (so no hover Reactions menu). Though you can adjust the timing of that with the post undo actions window admin setting.
The best way I find to play with test users is to have two browsers (Firefox and Chrome in my case) and have my admin in one and a test user logged in in the other (plus private windows as well if I want to use a third and fourth simultaneously).