Nice that Marko did a follow up by writing this blog post. A few months ago (in May), I also asked myself the question of "How plausible are our web analytics?", and I was able to see the same phenomenon [1]. Many people in my audience block client-side web trackers.
One option would now be to host a plausible proxy on-premise. But I didn't have time to try that out yet.
PS: We're paying plausible customers and our stats are publicly accessible [2].
-1: https://rugpullindex.com/blog#HowPlausibleareOurWebsiteAnaly...