Another update: I think there are in fact two different issues.
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When you reboot into 8.5.x after running 8.4.x, there is a DNS problem. This is apparent immediately after the reboot. I’ve seen this happen consistently several times, and made packet captures showing this. Fiddling with DNS settings seems to fix this bug and it doesn’t recur.
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some other more confusing and sporadic issue with devices on the VLAN not being able to see each other. Mainly affecting IOT devices, using UDP in my particular case a Philips Hue (ethernet) and an Apple TV (WiFI) serving as a HomeKit hub, which loses connection. Having an Outbound policy for the VLAN seems to be related. This one is very hard to debug, as it seems to only happen every few days, lasts for an hour or so, then recovers.
Since Firmware 8.5.2 is now in beta I’ve upgraded and will report back if I see any changes.