Interesting, what browser are you using? It looks like it's recreating the preview button for some reason, but I can't seem to be able to reproduce it in neither Chrome, Firefox, nor Safari. I can try to add an additional consistency check before adding the button and see if that fixes things, but it would of course be a lot easier if I could actually verify it myself.
Just so that I understand the whole flow, is this the interaction where the bug appears (with mobile UI off):
www.kbin.social/ -> Click on article -> Click on browser back button -> An additional button has appeared.