Excel can still use legacy “Move and Size with Cells” checkboxes — but can no longer create them

Your A/B package evidence is strong enough to report as a serialization compatibility defect. For an interim maintenance path, test whether copying one known-good legacy checkbox from an untouched workbook preserves its two-cell anchor after save, close, and reopen. If it does, keep a protected template sheet containing a small number of verified controls and have the maintenance macro copy a template control, then change only its name, linked cell, caption, and OnAction assignment.

Do not patch the OOXML of the production workbook as the normal workflow. A small packaging mistake can invalidate signatures, relationships, or the VBA project, and a later Excel save may normalize the drawing again. Keep the patched file only as reproduction evidence.

Submit the OLD and NEW minimal files through Excel > Help > Feedback > Report a Problem. Include the Excel build, the exact VBA creation code, the drawing XML diff, and a row-filter test showing the wrong control receiving the click. Ask specifically for xlMoveAndSize to serialize a true two-cell anchor or for a documented migration route in Excel 2024 LTSC.