I should think i t's probably due to either one or two things, maybe because of the holiday festiivties, they couldn't pay the banks on time, and also, there might be an element of staff training (if there is such a thing as staff training) involved, and that could also be why, they wouldn't have been able to pay you your benefits properly. I should most likely think, it's regarding the holiday festiviities this time of year.
Let's hope for you that as they say, you will have your benefit payment righted for the 8th January. And then thereafter fortnightly.
This is what I also t hought, absolutely nothing to do with, universial credit, and should it so be, a letter would have been in the post detailing for down the line, what was then to be happening, I think it is the case, of slight paranoia - here.