Thanks for your reply :)
First things first: I'm running a 2017 iMac, Mac OSX Big Sur 11.1, Music 1.1.2.28
I turn my computer off every night so this is freshly rebooted. Here's the search result as I see it on my Mac ... as you can see, no match found.

As requested, I tried the Music app on my phone (iPhone 7 Plus, iOS 14.2), and no match found there either ...

So then I tried my Apple TV (A1842, tvOS 14.3 18K561) .. same again, no match found.

You can see that in all three cases, I was given the exact same set of search results. So this doesn't appear to be dependent on the OS or device.
If it makes a difference, I'm in Australia, and have an ongoing monthly family subscription to Apple Music.
By using Google to find the album, and then adding the song(s) to a playlist, I'm able to play the tracks on any of my devices, so this obviously isn't a problem of access to the tracks. They're simply not searchable.
I did some digging around in the Preferences for Music on my Mac, and found something curious. If I enable Preferences->General->Show iTunes Store (which I didn't know even still existed, I thought Music replaced iTunes?), then restart Music, then search again ... it still doesn't show up ... unless I click on the "iTunes Store" selector on the right hand side of the screen. The album then shows up as expected. I don't know how to enable a search of the iTunes store on my AppleTV or iPhone so I can't confirm it there. But this does suggest that while the indexes for the iTunes store are complete, they are perhaps incomplete for Music? I'll leave that to you to investigate.
So my problem isn't solved, just worked around for now. But clearly given that I found two artists (or rather, could not find two artists) in the space of five minutes, despite the AppleTV automatically playing tracks from those artists, that's either remarkably bad luck or a sign of a widespread problem in Music.
Thanks, Lindsay