I haven’t seen this problem happen before, however I believe this might be an easy problem to fix. Before we go any deeper, you should verify the following:
1.Does anybody else on the forum experience the same problem? (If so, it may be #2) 2.Is this favicon reverting problem actually just cache? 3.Does the problem persist in other browsers? 4.Maybe try rebooting the server/rebuilding the container?
Seems like the error isn’t on the server-side, but it is on the client-side on Firefox. If you log into your forum with Google Chrome, will the discourse-sketch logo come back?
This seems like a bug on Firefox’s end, however, if this was a bug on Discourse’s end, then it should be addressed in the next stable update, since you seem to be running a beta version of Discourse.