Matrix not aggregating correctly (month/year)

This matrix visual worked fine for years. Its source was a SharePoint site that had full date and time, but after pulling it in to Power BI I formatted the date by Month/Year only (we only care about reporting out on a monthly basis). The data is supposed to look like this:

There was a recent corporate change where after Date X (actually August, 2021) the data came from a different SharePoint site. Since we still need access to historical data, instead of hard coding a change to the new site I moved the data from the two different SharePoint sites to secondary columns, both formatted as Date: Month/Year. Then the original column that was the  direct import from SharePoint site A became instead an IF statement, where "Before Date X" it's Dataset A, and "After Date X" it's Dataset B. That column (the IF statement) is also formatted as Date: Month/Year.

That was the only change, and yet now it seems to aggregating by Day/Month/Year, even though no part of any column includes Day in the format. What am I missing?