Sounds very much just like last November. I've asked a number of times both then and now if you can see it using the Fing app, largely because I'm not familiar with any Eero interface.
If there is a solid reason why you don't want to try the Fing app, please just say that rather than ignore that aspect entirely.
I'll also reiterate, this is so damn unlikely to have anything to do with the RPi itself, nor the Moode software distro, both of which are proven and used by many, including right here on this forum. Can an SD card fail? Sure it can, but that too is unlikely.
I've used countless iterations of RPi 3B, 3B+, 3A+, ZeroW, and 4B along with Moode, Volumio, and Gentoo Player as the software distro, and my units have never vanished like that, so I think this is a DHCP lease expiration issue of some sort caused by the router, however that doesn't explain why a reboot doesn't restore things, it should.
Can you try making a temporary connection via Ethernet to see if that makes this unit show up? If so, then it's down to a WiFi issue of some kind.
Also, on reboot, if you look very carefully at the front panel LEDs you should be able to see the green CPU activity blinking after any reboot, but because the black plastic bottom of your case is only semi transparent, that green CPU LED can be hard to see if the ambient light in the room is bright, or even just in comparison to the much brighter red Power LED, which is unfortunately right next to it.
What you should be able to see easily enough are the Ethernet LEDs on the backside of the unit, if you plug an Ethernet cable in and reboot. If you do so, also ensure that the SD card hasn't somehow become semi-dislodged.