Background sync does not filter out deleted items

[ChristophWurst]

Expected behavior

The background sync should filter out \deleted emails when it fetches new messages, just like we do when a message page is fetched.

Actual behavior

Deleted items show up if deleted with another client.

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[jmhbnz]

Encountered this issue when trying to use Nextcloud mail today.

This is the only email client I have used where messages deleted on other clients continue to display, I would expect the client to stay in sync with others as outlined above.

I tried to look for a setting I might have missed to keep things in sync but could not find anything obvious.