There seems to be a bug in Jira that you can't merge cells in a large enough table in issue description (not sure about comments):
1. Create a large enough table in issue description (5x100 filled with "0123456789" in each cell works for me
2. Save changes
3. Now go and try merging any 2 nearby cells
4. Try saving changes
Expected: all changes, including merging cells, are saved.
Actual: such error is popped up:

and all the changes are lost, at least I didn't find a way to back them up.
I'm not that sad about that I can't merge cells, I can live without that feature, but what is much more frustrating is that ALL the other changes are lost too if you tried merging cells and clicking on save. In my case, i have a large table, and I made a lot of changes in there besides merging cells. And now I have to that again because my changes were lost.
Note that neither clicking on the suggested link nor refreshing the page back up the changes or make any effect at all
Please push that to Atlassian developers as much as possible, I didn't find a way to report a bug directly to them.