CloudFlare Access with Tunnel and bypass posting 530

I have a number of hostnames running for a couple weeks now with both tunnels and access. I added bypass rules and an IP group as well as a WAF allow rule on many of them for UpTime Robot to do it’s thing. They are all configured with the exact same bypass rule on the apps, but ONE hostname keeps failing the monitoring checks with a 530, while none of the others are. There is nothing configured differently between the one failing and the other dozen hostnames. It’s driving me a bit mad as you’d think with identical configs and even using the same tunnel, the error would appear elsewhere.