Works when syncing the folder manually, or opening the quick-view which also triggers a sync.

The problem is that the interval sync is never executed. Every collection overrides the parent's cache policy, so the sync settings are effectively ignored.

The problem was that the interval sync doesn't work as expected.

First was the cache policy not appropriately applied.
Second is the interval sync only properly setup after restarting akonadi,
because a change to the cache policy of a collection doesn't reevaluate all child collections.

So after the latest fix it is just necessary to restart akonadi after changing the sync interval, and from there on things are working as expected.
This will not be further fixed as it is not an issue during normal operation.