Very weird indeed!
I tried switching it back to a select option by toggling to another option but there really isn’t anything to switch. It shows that the select option is selected but when you preview the page it shows as a multi select.
No in page builder it looks like it should work properly, in run time it doesn’t.
I moved the XML from one side to the other by copy/paste.
Here is my filter:
Production Render:
Sandbox Render (The way it used to render and production and I need to still work):
