I've hit a reproducible data-loss bug when editing fillable PDF forms.

  ▎ What happens: I open a standard fillable AcroForm PDF (89 form fields: 66 text + 23 checkbox), add one text field via Prepare a Form, and Save. On save, Acrobat silently drops ~20 of the 89 fields — specifically all the checkbox/radio (button) fields (e.g. 

  ▎ agency-type checkboxes, AM/PM toggles, accept/reject options). The text fields survive; the buttons vanish. There is no warning. The resulting file is broken for downstream automated form-filling.

  ▎ The form is clean — I verified at the PDF level:

  ▎ - Not an XFA/hybrid form (no /XFA).

  ▎ - No security, encryption, or certification (no /Encrypt, /Perms).

  ▎ - All 89 fields are well-formed (each has a proper /FT, /T, and /Rect).

  ▎ - NeedAppearances = true in the AcroForm dictionary.

  ▎ My theory: on save, Acrobat regenerates field appearance streams (because NeedAppearances is set), and the appearance regeneration for the checkbox/radio fields fails — and instead of erroring, it drops those fields.