My shared developer user cannot update data values in a checkbox field, but owner can

My shared developer user cannot update data values in a checkbox field, but owner can

In a form called "People" I have a checkbox list named "Current Staff Roles".  It has worked before properly, and still does for the owner of the app, but my shared developer user suddenly cannot make any selections made in that checkbox list stick, i.e. checking new boxes and "updating" does not cause the changes to appear in the data view.  But it does work well for the owner.

I have shared the app w/ support.   The owner is w21admin and the shared developer user is datazen1.

I'm already guessing you're going to tell me to delete the field and recreate it, and my guess is that'll solve it --- for this instance.  But I'm more interested in you (ZC dev) understanding how this change in behavior of the field can happen in the first place.  If this app was already in full production mode (i.e. being used by my org) and this happened, it'd be horrible and risk the loss of data.  So, please take a look and try to fix whatever might cause this.

Please advise.  Thanks.