When you use core's deletion interface (e.g. http://127.0.0.1:8080/w/index.php?title=Topic:Sc74aet4uq42fd4y&action=delete) it appears to work (except for a useless message analogous to T90971), but the topic is still accessible with the normal URL, and visible on the board.
We should either block this, or map it to Flow's internal deletion. I'll put up a quick fix to block it.