It seems that it only effects local clients using Android. After a couple of hours messing with this, I tried to watch one of the titles on a Moto X4 but with cellular data - and it played successfully. @ChuckPa is right - it is a DNS issue.
I am using a Pi-hole + local unbound resolver on my network, which prevents DNS rebinding without additional configuration. I added this to the unbound configuration in the server directive and it restored functionality to the Android clients:
private-domain: "plex.direct"
I could not accomplish this with Pi-hole’s DNSMasq alone, I tried using:
rebind-domain-ok=/plex.direct/
