Well so I can’t reproduce at all.
Exact same query but different result
"size": 1,
"directPlayDecisionText": "Direct play OK.",
"generalDecisionText": "Direct play OK.",
And everything works fine.
The only difference is that in your logs you are connected via the Plex relays and not directly. They maybe prevent playing the media in that case if the bitrate is too high. Unfortunately since the relay will nearly always answer there’s little chance Symfonium return to direct IP automatically 
Would be nice to had logs of why Symfonium switched to that one to I can better handle a try to return to better connections.
Anyway removing the host and adding it again will almost certainly fix the issue, I’ll need to figure out the relay thing