I’ve seen this a couple of times in the past few days.
I’ve got a UserStream connected and have seen at least 5 mentions delivered over UserStream but across all apps on the REST API on /statuses/mentions those mentions are missing?
I can’t see any obvious reason for it, they not are blocked, muted, the tweets can be accessed if I actually try to load it manually.
I’ve got some example tweets I can DM someone if it helps debug