seems like a problem in webkit. I have the same problems with Chrome and Safari. This appears when there are lines of a certain height, except for some in the center. Only the latter will scale if there is another column with a higher number of rows. in the horizontal plane, this problem does not appear.
You should use another table in the same cell in columns 1 and 3 and avoid rowpan
<table border="1"> <tr><td><table height=100%><tr><td height="100%">COL 1A</td></tr> <tr><td height="100%">COL 1B</td></tr> </table></td> <td><td>COL 2</td> <td>COL 3A</td> <td> COL 4<br/> COL 4<br/> COL 4<br/> ...
Also, you need to manage the borders for the internal table with css
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