Any idea what this is about? I doubt Proton has that many trackers, so I was thinking maybe the individual emails? But those would just be triggering when I click on links, which I thought Proton stripped anyways, and I wouldn't think /e/os would attribute those to Proton either.
I dont have an answer but perhaps relevant context: google analytics is by far the most used solution by marketeers, a lot of newsletters have code embedded so the sender can see if it the email is opened and if so what links are clicked. Opening an email is enough to pull the tracker though, since that is the action that makes the code start running. I would say it is much more likely the trackers are from the emails, but since i'm not a developers I don't know how much analytic software like this is used by them. Also I don't know how effective proton is at stripping them. You could consider using a forwarding service, but that can be a hassle too. I use addy.io, and also have good experience with duckduckgo's service. If you have a paid plan from proton i believe they offer this service too, but maybe it isn't more effective at blocking trackers than what you have now. They did acquire or merged with SimpleLogin, but don't know how much of their service is already implemented in the regular email service.