Hi Ariel,
You can compute the exact height of the merge field by implementing QureyRowHeight event with the help of AutoRowHeight functionality. Can you please share more details for your code snippet or sample on how you have customized in your application for cell merging and auto row height implementation, thereby we could analyze and update you with the appropriate details at the earliest.
Also please share the replication procedure for the user interaction to reproduce the reported issue.
Regards,
Deivaselvan