I found 3 things:
- Your syntax isn’t correct. You need to put validation rules as a param of meta box, not under
fields. Here is the correct code: https://ghostbin.com/paste/gnch6
In the rules and messages, because the field is a checkbox list, its checkboxes has name
obh_member_membership_tags. We have to use the input name, not the field ID. See the code in point 1 how I fixed it. I should have mentioned that clearer in the docs.
There’s also a bug with Meta Box, I’ve fixed it on Github.
Please try it and let me know if there’s still any bug.