I don't see this, which skin do you use? Does it always appear or only when you change the skin at the same session as you cast which could indicate rather a problem when closing MM5 while casting and not storing the used skin value to the persistent.json properly?

If you can still replicate then catching this in debug log could shed some light on it, see item 4b here: https://www.mediamonkey.com/forum/viewtopic.php?f=30&t=86643

Every time you play the sound via "Google-Cast" and then restart MediaMonkey 5.01.2433,
the design is always changed to the "Material design (dark)".