I talked to my hosting company today about this, they said it wasn’t them (I thought something might have happened while they were updating my mod_sec, but no).
Anyways, if someone is kind enough to take a look at this for me, I made a page of screenshots (in progressive order) of what happens when the issue occurs. Cannot believe I’ve spent four days on this! I’m sure I’m just having a newb moment…?
https://www.cherryera.com/screenshots/screenshots.php