I'm having similar, possibly related issues, in WISP mode:
When the Chromecast "disappears", I can still ping it from the router, and it still appears under "Network>Wireless>Associated Stations" and the Chromecast itself doesn't complain about lost connectivity. But I can't ping it from any other device on the LAN, nor select it as a casting target, until I either go to "Interfaces" and restart the LAN interface, or totally reboot the router.
I feel like it could be Multicast/IGMP related, but that area of networking is frustratingly cryptic to unpick for me...