it seems quite specific to me that there is a calendar app installed that automatically causes “manual” syncs
It’s not only calendar but also contacts, a few days ago I had a DAVdroid IO error notification from contacts sync even though I didn’t even open the contacts app for a few days. (Google apps, both contacts and calendar).
Google doesn’t care about our usecase, they only care that your data ends up in the Google cloud asap. So I think it doesn’t matter what SYNC_EXTRAs the apps use or if that usages aligns with Google’s documentation. What matters is if DAVdroid meets the user’s expectations. Which it apparently does for thousands of users except two ;-/
In my case, I must say DAVdroid betrayed my expectations by providing a “sync interval = only manually” setting. Thus I chose to set up my radicale server in the most simple way to sync via http with plain text auth. I think if DAVdroid allows to run syncs at random times controlled by random apps then it should consequently enforce to use strongly encrypted and authenticated connections.