Win 10 21H1, Chrome Version 92.0.4515.107 (Official Build) (64-bit)
After I post a reply in a thread, the reply editor does not fully load. Refreshing the page does bring it back.
Edit Testing on Firefox ver 90.0.2 (64-bit)
Edit 2... Same result as when using Chrome. But an additional piece of the puzzle. The editor loads correctly if you save an edit to an existing post. This only seems to be happening for the original post.