ICloud – iPhone SE says “connect to wi-fi or cellular to play music” when I AM connected to cellular

I can't figure this one out, and I don't often listen to music while out & about so I only just noticed it recently.

iPhone SE, iOS 11.3. I have both Apple Music and iCloud Music Library subscriptions.

If my phone is connected on the Cellular network, I am unable to play certain songs on a smart playlist that is on my phone. Anything from the "For You" or "Browse" tabs in Apple Music will work just fine, and SOME songs in the very same playlist, will also work fine. But most will not play at all and simply say:

"Connect to wi-fi or cellular to play music"

Obviously I AM connected to Cellular, so this is very confounding. The songs in question are all in my iCloud Music Library, or Apple Music. Some report an iCloud Status (in iTunes on my Mac) of "Uploaded", some "Apple Music". There is no pattern that I can discern, all play fine on my Mac but it's completely hit or miss on my phone.

Again to be clear, some music will play just fine. But some will not for no obvious reasons. This means it is unrelated to needing to enable cellular data for Music, or anything like that (though I went through all those steps anyway just to be sure).

So far, I have:

• Rebooted the phone
• Toggled every place there is to toggle related to using cellular data for music
• Quit & relaunched the Music app
• Googled and found nothing related to this exact issue

I'm stumped. I don't often listen to music in my car, but today was one of those days when I wanted to and… I couldn't listen to what I wanted to listen to.