Oh, I should also mention - even though I had it set to 64 when I did the previous tcpdump, after I changed it to "Auto", it still showed the same NoPrefixAvail message. I just didn't reply to say it was doing the same thing. So, it's not that it was configured for auto but requested 64 - at the time I did the initial tcpdump, it was configured for 64. But then I changed it, and still saw the same message.
However, tonight, after work, I'm going to reset 19.07 to the initial factory settings and then resetup my router, as standard as possible (of course, doing things like setting up the WiFi SSID and password again), to verify that does not fix it.