Seems odd. Has the source device, feeding the TV, or the TV itself received an update? Neither should be changing your setup, but ‘should be’ hasn’t always been true for me.

What ‘show’ is your test? Where are you watching it from (what service)? Have you tested with another show?

Was watching ESPN on YouTube TV. And just like that, Arc is playing in Dolby multichannel PCM 5.1 again. Never mind

YouTube is somewhat famous in this forum  for claiming formats that they’re not sending. And honestly, much of what they broadcast on ESPN is suspect too. Seems like sometimes, their associated broadcast from Sky Sports (F1] is in Dolby Digital, sometimes it isn’t. No idea why, but my contacts with ESPN no longer work there. 

And, just to further confuse things, the CBS affiliate (as I recall) in Austin TX used to broadcast the Cowboys pregames using a Dolby Digital carrier. Unfortunately, the signal they were receiving from the Dallas area was in stereo. So my PLAYBAR was trying to play a stereo signal, but while reporting (appropriately) that it was receiving Dolby Digital. Rare event, I’ll grant you, but mistakes are possible. It’s frustrating. 

And it was an interesting discussion with an engineer at that station, who seemed surprised that a member of the public would care, and be aware of the nature of the situation. It was a few years ago, now, but interesting. Back in the early-ish days of Dolby Digital, at least in Austin. 
 

Edit: never had that issue when I lived in Burlingame/San Mateo, but then they wouldn’t be caught dead broadcasting Cowboys pregames ;)