Why is it, when all Cloud Connectors in a given resource location reboot or lose Internet connection, do connected sessions get stranded forever? I would expect that once a cloud connector is back online, the VDA is re-registered, that a user could reconnect to their disconnected session like the old days with a NetScaler.

With gateway service ? if so it will be because the HDX connection is being proxied through the cloud connectors if you are not using rendezvous

Yes with Gateway Service. I can understand in the moment that Cloud Connectors are down that no ICA traffic passes, but why after when they come back up can sessions not get reconnected? The session still exists on the VDA. Citrix DaaS is still aware of the disconnected session. An admin can see it in the cloud Studio and Monitor.