Sometimes if you don't close an url tag (accidentally) the whole post is removed/not visible when rendered, but otherwise it can be edited from there (so it's available). Is this normal behavior?

This board has many issues related to URL parsing. Refer to:

https://board.flatassembler.net/topic.php?t=19228

Unfortunately, T.G. has already stated that he would not have time to fix such issues in the foreseeable future.

So, just enjoy what we still have and stop asking for more.