There are many posts in the Google help communities and Reddit about phones like the 4a, 5, and 5a still stuck on November. I'm thinking there may have been a problem with the December update (since there has coincidentally been an uptick in posts about those phones giltching or even bricking after the December update), but Google hasn't made any official announcement yet. The 6a hasn't been mentioned quite as frequently, but it might be included as well. Since January also seems to have been paused (at least for some people, like me on my 7 Pro), I'd just sit tight and wait for them to sort things out.
Bugs happen. They happen on Samsung updates as well. I'm increasingly of the opinion that Google (and Samsung) should strongly consider moving to an every 2-3 month update schedule, because it seems like this monthly pace of updates increases the chance of introducing bugs. Obviously, if there's an urgent bugfix or security flaw that needs to be patched, then get that update out ASAP, but otherwise, I'm guessing that a lot of these things aren't super-crucial, and could wait a little bit.