The YouTube TV issue isn't unique to Monumental, but the way they ran the banner constantly for the whole game was pretty unusual. USA Network ran a similar streamer across the top of the picture earlier today during the Ryder Cup broadcast.
When I saw that notice last night, I made sure to log into the Monumental app on my iPhone and iPad. A subscription is included if you have Caps season tickets, so I wanted to make sure I had it active. I'm going to have to see whether there is an Apple TV app for their service.