I spray alcohol from a spray bottle on mouthpieces while they are attached to my melodicas (for disinfection purposes). Alcohol often gets all over the ends of the melodicas and the keys. I wipe it off. Have not had any problems yet.
Every week or so, I collect all my mouthpieces and throw them in the dishwasher. I worry a bit about exposing 50-year-old plastic repeated to modern soaps and high temperature water and drying cycles. However, after nearly a year for some of them, I have not experienced any problems. However, none of my mouthpieces have glued joints.