Brilliant! That was exactly the problem, and the list is working now.
Ironically, file permissions were among the very first things I had checked, because of experience with MM2. Where I failed was that I didn't look in the /var/lib/mailman3/templates/ directory. We aren't currently using the custom templates feature *per se*, and I didn't realize Mailman was still creating per-list templates by default. Lesson learned.
I was so careful to not use root during the list migrations, but I must have goofed on that one list. While I was fixing this, I checked the others as well, just to be safe.
I've been whacking my head against the proverbial brick wall for far too long on this. THANK YOU so much for the pointer to the one small detail I had overlooked.
Best regards, and my sincere wishes for a fantastic week, Scott