Fix Word Add-In Spellcheck

I have a custom Word add-in that hosts a rich-text box, and its spell-checking feature is not behaving as expected. The usual “squiggle” underlines is not removed but shifted to part of the letters when a word spelling is fixed, yet I know Word’s proofing tools work fine outside the control. I need a developer who is good at Vishual statio C#.

Key points
• The add-in must continue to load smoothly across the Word versions my users run (they span several releases, so please keep the solution version-agnostic).
• Any fix should preserve the existing UI and functionality; I only want the spelling behaviour to work as expected.
• Deliverable: updated source plus a brief note explaining the cause and the steps you took, so I can maintain the code in future.

If you have solved rich-text or RtfBox spell-check issues in Word before, you will recognise the pain I’m facing. Let me know your proposed approach and how quickly you can turn this around.