Hi,
I’m a little bit ashamed to say its the first time I completely removed the keycaps of my UHK60v1 for a clean and run into the same problem like shown in the pictures. I had the same observations as @DavidHarkness there. The surface of the keycaps looked really smooth. After cleaning it is more rough and looks exactly like the keycluster keycaps.
I don’t belive it is scale buildup or anything like that. I used osmosis water with the same problem. The residue is not reacting to acid and cleaning it “polishes” the surface, but it doesn’t matter if its microfiber with water or acid.
The residue is more “rubbery” and “gooey”. Since it sits in between the rough surface, it is hart to clean and only scratching it with my fingernail seems to really work.
The tip with the oil:
This only works if the residue is left “intact” on the keys. It polishes it, makes it “shiny”, but doesn’t “cleanup” the keycap. This will be a easier solution for ruined keys, but beware: if you have cleaned one keycap already, scratched the residue, etc. polishing it with oil won’t lead to “clean” results.
Cleaning with only warm water and a little bit of dishsoap:
Rubbing the residue of on one half:
“Smoothing” with oil, slight improvement but will whiten again if the oil is rubbed of by using the key:
I also tried dishwater tabs, acid and keycap soup.
Supplied for your entertainment, don’t try this. 100 C should be fine for PBT, still it didn’t help with the residue, so no need to try this 
Isopropanol also doesn’t do anything. And if you love your wooden palmrests, you should stay away from that stuff anyway. ( Wood wrist rest re-coating )
This is the current state of my keycaps after trying to clean with water + soap.
Note the Keycluster keys.
Note Keys 3 and 4.
4 is cleaned completely. 3 is “polished” with vegetable oil. Looks fine in this picture, with a slightly other angel you can still see the residue left.
@mlac I guess it might be hard to verify anything about injection mold release agent stuff used, maybe still worth a try.
I can’t think of any other good solution then to cleanup every single key once.
While I understand that you most likely will not replace whole sets of keycaps after x years, it is something which should not be a consumer problem.
This is frustrating as f*****. Would I still buy a new one tomorrow if this breaks? Most probably yes.