When a field on a form is of type Names and Computed when composed and marked as Show online status, the content can be deleted from the field in edit mode. If you remove the Online status mark, the field works as it should.This BUG is in all versions 8.x
Subject: known issue
As mentioned in the other forum where you posted this question:
This is a known issue, spr #: YMAA6BFBMF, which has been deferred for a couple of releases.
I just opened an 8.5.1 codestream doc for the spr. I would recommend that you open a PMR against that bug #; the more customer weight, the less chance that a bug will be deferred.