This has happened before a couple of times and there are old threads about the previous slowdowns (too lazy to look and post links, but you can search). I think the common observation on those occasions was that MUFG duplication "hung" for a day or two, and then the missed duplication days would be made up by multiple duplications all at once.

My own thought on this is that it's akin to the well-known tendency for Guardian counters to "hang" for a day or two, then suddenly catch up, but that's just my own theory backed by very little research at all.

However, I have 13 MUFGs with various payloads on board, 8 of which I keep at 97 items, some with around 60, and some in the 30s (I'm a little OCD about keeping the same types of gear in the same capsule, so that accounts for the disparity in capsule population). Since I noticed the most recent slowdown, I've started removing 2-3 items from the capsules with 97 items on days when nothing duplicates just to make sure I've got room for multiple duplications when it does start paying out again. A couple of times, I've gotten as many as 6 items duplicated in a single day following a day or two of duplication drought, and on one occasion, one capsule went from 93 items to 100 on a single day (I should have bought a lottery ticket that day).

That little bit of informal observational research does tend, in a small way, to support the theory that something's hanging MUFG duplications in much the same way as the guardian counter hangups, but again, that's just one person's informal observation not backed by any sort of organized research.

TL;DR: Yet another theory about this is phenomenon which may well be complete bullshit.