When selecting a time period of "Month" and selecting to compare to "Previous period" for some months causes Matomo to compare to the same month.

Have tested and so far it occurs when selecting the 31st of the month for December, October, July, May, March 2020 e.g.:

Link where we can see an example: https://demo.matomo.cloud/index.php?module=CoreHome&action=index&idSite=1&period=day&date=yesterday#?idSite=1&period=month&date=2020-12-31&segment=&category=Dashboard_Dashboard&subcategory=1&comparePeriods%5B%5D=month&compareDates%5B%5D=2020-12-01