This is what it looks like from mp3tag:
Your Action #46 is not the same as you highlighted in the screenshot.
Sorry to say, but this is far too complicated to give you any solid answer to your question "What's causing that/what's the fix?"
The complexity in your regular expression collection is so high, that in my view, it is no longer maintainable.
Maybe others can still help you, at this point I have to get out.
It's frustrating, because if I have two actions run apart then it works correctly.
You did just make me discover that you can now create preset action groups (new feature since I last used mp3tag), and I did create an action group with these two that works. It would be great to be able to have one action instead though.
I wish I could understand the code better. Shame there isn't an MTA file viewer/editor for those who have long lists of actions.
It's interesting how running the TITLE (ARTIST) action first results in TITLE (ARTIST, ARTIST & ARTIST) and then running the other action does not change it to TITLE (ARTIST & ARTIST & ARTIST) when that's what happened trying to combine them.