I saw this in a table that I imported from Word, but I’ve been able to recreate it many time with just inserting a table in a text page.
Create a new project
On a text page, add a table with 8 rows and 2 columns
In the right column, add the letters A-H, one in each cell
In the left column, select the first two cells, merge them.
Do the same for the next two, etc.
You should end up with 4 cells in the left column, 8 in the right.
In the left column, number the cells downward: 1,2,3,4
Click on some other element in the project, or add a file, whatever. (this may not be necessary)
Now exit the project.
Reload the project.
Click on the page with the table.
The first row contains cells 1 and A.
All the other rows are single cell, no merged cells, and the other data is gone.
Note: examining the RTF file in the disk shows that the data is still there, it just not displayed correctly.