I narrowed down the problem, by rebuilding the form field-by-field. I used several "Plain Text Content Control" fields along with a "Drop-Down List Content Control" field and a "Date Picker Content Control" Field, and when I inserted them I used the defaults, restricted the document to "Filling in forms" and started enforcing protection. All of these fields worked as I wanted them to. I then went back to each field for customization and after each step, I tested the form with protection to see if the same problem was occurring. The problem surfaced only when I checked "Remove content control when contents are edited." Apparently, this field does not do what I thought it would, which was to remove the content control once text (and by "text" I mean more than one character) was inserted. So what is the effect when a user selects that option?