We noticed this playing with data: https://stats.wikimedia.org/#/wikifunctions.org/reading/unique-devices/normal|line|2-year|(access-site)~mobile-site|monthly
Mobile site data for Wikifunctions is not available, but it's available for other projects. And it's there in Druid: https://w.wiki/Ahw5. Since Wikistats uses Cassandra for the mobile data, I figure the problem must be in the way we handle domains as part of the load code:
And indeed we just load the domain with no manipulation in the druid load:
So this all makes sense, but there's some kind of bug.