Update (9.11.21): I take back what I said below. Playlists are pausing after one hour, now. Sometimes after only playing one song. If this is an Amazon issue, I'm out of luck, because they won't claim responsibility. So, I'm hoping an Roku agent will respond to this, and request detailed information about my device (e.g. serial number, device ID, Software OS/version, tracker number). That's the information that will help them REALLY tell if it's their issue, or Amazon's. Every time I've had issues with Amazon in the past (Prime Video not working, for instance), when Roku requests the information I've just alluded to, the problem has been resolved.
Soooo, my little test worked . . . sort of.
I selected a playlist of 27 songs, and set it to repeat. It paused after about 3 hours on song number 17 in the list, and asked if I was still listening. To get an accurate result, I'll have to try another playlist and see how long it goes before it pauses. I suspect it will definitely play longer than a station, but maybe not 3 hours?
Also, after doing an online search for this behavior, I've seen several comments dating back a number of years about stations only playing for an hour on Amazon Prime Music before pausing and asking if you're still listening. So, there must also be a control set for Playlists that only allow them to play uninterrupted for 3 hours, or so, before pausing. Ostensibly to manage their streaming bandwidth? I think this is an Amazon practice, not a glitch. Still, it would be nice if they'd be more transparent about it.
Side note: Uninterrupted play time isn't listed as a benefit in any comparison list I've seen between Amazon Prime Music and Amazon Music Unlimited. So, if Amazon Music Unlimited does include uninterrupted play, they certainly don't advertise it.