When sending a PM, it looks like all line breaks are ignored, mashing all paragraphs into a solid block of text. Other message formatting seems to be ignored as well (e.g. lists, horizontal lines, etc.).

Not a huge deal, but it makes messages harder to read/write in a decent way.

It's supposed to use markdown. I think you have to use double spaces at the end of a line so it counts the line break. :P I'm not even sure that completely works correctly though.

If that's the case, that information should be provided to the user when they're typing their message, and stuff that doesn't work at all should be removed from the formatting options.

Totally agree.

I tried double spaces; that's ignored as well. I finally just added a bunch of invisible characters to break things up. (https://www.invisible-character.net/)

Also, every time I made an edit, the site errored out trying to reload the message and I had to return to my inbox and find the message again, which doesn't seem great.

Not blaming you, or anyone in particular; just kind of ugh. Hang in there. :)