Answered
Hi @dublove ,
opened your sample InDesign document and did some tests.
Yes, I see the same issue when unmerging the cell that reads "2018".
So why does this happen?
Explanation: in the Character Styles panel the character style "aa" is selected.
Screenshot after I opened your document, navigated to page 2 and opened the Character Style panel:

Solution:
If you select the "None" style in the Character Styles panel and, as a second step, unmerge the cell, all is working as expected.
Regards,
Uwe Laubender
( Adobe Community Expert )
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