Once again I'm encountering the issue that my custom fields won't save. I already increased the max_input_vars from 5000 to 10000, no change.
I have 4 date fields that I'm trying to update, and they always seem to revert back to their old value, which must be stored somewhere I don't know. If I try to delete the values, they get deleted as expected. But if I try to change the value to something else than the original values, even after having deleted them (and therefore the database should have no 'memory' of the old value), they revert back to the original values.
I tried modifying the value directly from the 'custom fields' pane, same results. The "save field value" checkbox is checked in the fields settings.
I have no idea where that 'old data' is stored as it keeps coming back. The only change (other than updating custom fields) I made to that page recently is to update the permalink, in case that's relevant
I import the field group to my demo site and add some values to the date fields and save but don't see that issue. You can try to remove other field groups, deactivate all other plugins, keep only Meta Box and MB plugins activated, switch to a standard theme of WordPress and check this issue again.