There was very recently an issue with the DNS server for the plex.direct domain. It failed to flag its responses as “authoritative”, which caused some DNS forwarders to disregard them.
But this should be solved now.
I’d try to use a known “good” DNS server as master, like 8.8.8.8 or 1.1.1.1 instead of the one of your ISP.
Also think of configuring pfsense to not perform DNS rebinding protection for the domain plex.direct, for the sake of all clients in your home network. https://support.plex.tv/articles/206225077-how-to-use-secure-server-connections/
