getFormMsgSubject contains a piece of code that is just a clone of un_preparsecode... almost.
The only different is the callback of the "html" tag part.
One is:
https://github.com/elkarte/Elkarte/blob/11bfe9b7dcf6fd0cb1e6449bdda22fb8bb7bc216/sources/subs/Post.subs.php#L352
then other is:
https://github.com/elkarte/Elkarte/blob/11bfe9b7dcf6fd0cb1e6449bdda22fb8bb7bc216/sources/subs/Post.subs.php#L1654
I'm tempted to use un_preparsecode in getFormMsgSubject as well.
Do you see any problem?
https://github.com/elkarte/Elkarte/commit/b122aab600369c5a8a6c2223ab22d192af6cd329
Seems like something that should be tested.
I did some quick tests, but TBH I was not entirely sure what I was expected to test... :-\