I was testing the new comparing feature inside the calendar and I found this.
I expected a compared dashobard of 2018-11-05 day period and 2017-11-05 day period but query is ran with 2016-11-05 CompareDates and ComparePeriods = range.
Then I've expected I can compare day periods and not custom ranges.
This means to me Matomo is always calculating a range report. Not using pre-archived periods.
Note: «Compare to: previous period» works like expected
When the query is done, if I open it in a new session (incognito for example), «compare to:» previous period is selected inside the calendar-dropdown, but previous year query has ran.
You can have a try in demo also.