One thing which is also very important to PMMA is which what kind detergent it is cleaned. Some cleaning agent does make the plastic brittle and so called micro cracks will form, especially where acrylic is bent or cut. With some fluids the arcylic glass will turn white and milky faster than if cleaned only with water and mild soap. What I’ve seen in some fluorescent street lights is that sometimes plasticizers in cables will turn the PMMA bowl white. 

I hope that dish soap don't get the PMMA brittle. Actually my NISKO PL-S 9W bulkhead, seems to be made from PMMA if not polycarbonate, as I have it long time above my bed, and still didn't browned.