IOS Mail does sometimes delete instead of archive with Google Inbox account — why

I'm using Google Inbox as my primary email service… sometimes using Inbox as app… sometimes using iOS mail. Although I've configured iOS mail to archive everything (I don't want to delete at all) some emails – not all – are moved to trash instead of being archived. I could not find a clear pattern for this behavior and think of it as a bug with iOS mail. Does anyone of you have that behavior too? I'm looking for a fix… so far without success. I still need to check Inbox' trash folder every 20+ days to avoid auto deleting those emails after 30 days. That's annoying…

Info: related to iOS v10.2.1