I found a bug in Meta Box Group somewhere between version 1.3.4 and 1.3.11
group-fields.php file the filter of line 83 was changed from
rwmb_raw_meta and that makes my group fields not load the values when it comes to select fields. They are saved in the database but they are not shown in the select fields.
I couldn't check the exact version on GitHub because I guess the repository is private.
Can you review it?