Days with no visits recorded but all pageviews

I’ve recorded a strange month during December for one of my sites. On some days it recorded no visits but at the same time recorded the expected number of pageviews, which proved the site wasn’t offline.

It happened several times. So far no indication of the same in January.

Here’s an image of the visits and pageviews

This have never occurred previously and the Matomo/Piwik has been recording the site for years with consistent stats.

Any ideas?

This hasn’t happened so far this month, but it would be quite good to try to find why it happened previously…

I notice that the missing visits after 7/12 have now appeared following the update to v3.13.1. The 7/12 dip was me trying to work out what happened and doing a DB restore. 4/12 is still a mystery though.