TL;DR:

1. I have an Excel file, .xlsx, contains some spreadsheets, and many merged cells in each spreadsheets. Opens in computer A, works well. A has an Excel of version 2211

2. The data in it is written by openpyxl package of Python.

3. I copied the file to computer B, the excel version is 2209, opened it, all merged cells are broken up (unmerged, split up). Not only this, Excel crashed 3 times before opened it up.

4. Firstly, I though it is openpyxl's fault. No, it is wronged.

5. Ended up some bugs related to this issue were fixed in versions above 22.09.

6. Update B's Excel (of course the whole Office actually), bump up to 24.11, opened the same file, all merged cells are back!