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.
Right, you don't have to interact with an email to have trackers. Just opening it is enough, and trackers can be embedded in images or other elements.
I don't know much about Proton, but it may have features to prevent loading those elements.
I use Thunderbird, which optionally blocks those elements, and can work with Proton on some devices.