I’m trying to move a field between tabs on a custom record form, but it doesn’t SHOW under that tab in the Move Form Elements tool. Has anyone encountered this before?

Try turning it off the record completly and then apply to forms again. Set the subtab correctly before you do it and it should actually land on the correct tab anyway then.

I've encountered this with certain native fields, they don't seem to be moveable. E.g. preferred vendor on the Group/Kit/Assembly form. But I don't know of any solution unfortunately!

I ended up giving up, deleting the field, and creating it again