The new firmware 4.6.417160 fixed this issue for me!
I have this same problem with my device too. I will use Airplay on it, and then the next day, I'll need to cycle power to get it to "show up" in the airplay device list on my apple device. This makes the device almost unusable for me, as I have the WiiM in a location that is not easily accessible.
Seems to be resolved with 4.6.417160 for me as well! I did have to restart the device one more time after the update, but it's been working since then.