I've been having an ongoing issue with two Eeros (6E gateway + 6E acting as a wireless extender). Any battery-powered (sleepy) Matter-over-Thread device that parents to the bridge goes unavailable in Home Assistant while still answering ICMPv6 pings... i.e. it's reachable at the IP layer but HA can't sustain the Matter subscription across that path.
Physically disconnecting the bridge forces the devices to re-attach to the gateway, and they immediately work and stay stable. The gateway is closer/stronger (~10 ft vs ~20 ft), so I don't know why they attach to the bridge in the first place, or why they never reparent... though I think that's because the Thread link to the bridge stays "up" from the device's perspective... so nothing triggers a re-scan.
My one mains-powered Aqara wall switch (a Thread router, always-on, not parented) never has the problem (and it's in the same room as one of the sensors that becomes unavailable), so it's specific to sleepy end devices.
Anyone encounter an issue like this before? For now I guess I'm just resorting to disconnecting the bridge for a minute when it happens. It's not incredibly frequent... I can have these devices up for a week at a time.