There are several things weird about drafts and the “reply as topic” feature at the moment. I was planning to make a video about a particular bug I thought I had a clear step-by-step reproduction ready for, but another bug turned up as I was doing it.
Here’s what I know for sure: Clicking “reply as new topic” when your post form is folded down, you’re gonna have a bad time. More specifically, it’ll add the newly quoted post below whatever content you already had in your post form, as opposed to overwriting it.
Furthermore, there’s some weirdness with stored drafts that I haven’t quite wrapped my head around yet. But bad things happen if I have several drafts open at once. I tried to keep that particular bug (pulling in a previous draft, unrelated to the current post) from happening in my video by closing all tabs, logging out and logging in again to operate in one tab only, but it seems that draft is saved on your end and not mine (I didn’t delete cookies but I was in incognito mode).