Since about 2 or 3 weeks ago, I have two categories in the stock client: Multimedia and MultiMedia. As far as I can see exactly the same apps appear in either of them. This happens on two different devices, so no “cosmic ray” 
You have 3rd party repos? They can define their own…
Seems like the cosmic ray is IzzySoft.
Yes, I have Izzy enabled. Reported here: