So it boils down to the fact that `YouTube Music != YouTube`. In that case you could be right about the user's expectation.
I for one don't use YT Music, but to use YT. Then again I don't use Pandora or Spotify as well, but do listen to music on YouTube (non-music). In my case, I'd expect the search to be executed in the context of YT, but that's what the defaults are there for. I'd choose YT (non-music) as default, if that's possible, or YT Music if i'd care.
Yes, somehow it does make sense that it selects the app which is set as a default, even if I would expect it to perform the query in the opened app.
Can it act upon "Open Song/Performer in Pandora/Spotify"? What's so hard about it? It all doesn't make sense to me.
I'd expect it not to query in YT Music but in the app which is currently open, which is simple YouTube. No, it feels like Google shouldn't have the right to expect YT Music to get launched if it is not set as the default app.