I was able to reproduce your result using power tables, and it really does seem like a bug. When I use system.tag.queryTagHistory with a returnSize parameter of anything other than the default -1, my resultant columns are misaligned by 1 value:
In the above example, the table on the left has a return size of 10, and the table on the right has the default return size.
Query 1 code:
system.tag.queryTagHistory(paths=[path1], startDate=startTime, endDate=endTime, returnSize=10, aggregationMode="LastValue", returnFormat='Wide', noInterpolation=True)
Query 2 code:
system.tag.queryTagHistory(paths=[path1], startDate=startTime, endDate=endTime, returnSize=-1, aggregationMode="LastValue", returnFormat='Wide', noInterpolation=True)