You would need to post a sample document that demonstrates the problem but I would guess that your issue is caused by the Table Style settings.

Tables always have a table style associated with them. And each table has a series of options that turn on/off conditions for that table style. There is an on/off setting for the First Column condition in the table - often this setting would be making the text in the first cell Bold. If I was to split the first cell, this setting could make the new first cell bold whilst the new second cell (which now isn't in the first column) would not be bold.

So look at the table style applied and check which table style conditions are turned on.

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Andrew Lockton
Chrysalis Design, Melbourne Australia