Thanks, @wbk. Happy to report that this is now working. It was partly your suggestion to check the OpenWRT logs that set me off in the right direction.
The DNS queries doen’t show up in the OpenWRT logs by default, so I hadn’t noticed anything when I checked earlier. But your question made me look again and I saw in the OpenWRT documentation that you have to enable DNS logging by SSHing into the router and entering:
uci set dhcp.@dnsmasq[0].logqueries="1"
uci commit dhcp
service dnsmasq restart
https://openwrt.org/docs/guide-user/base-system/dhcp_configuration#logging_dns_queries
Them you can see the entries in Status > System Log
Also, for anyone reading this thread later, the logs get filled up quickly, so you’ll want to check immediately after doing anything.
In my case it was that a CNAME entry I had set up earlier when I was trying a different tact with my setup was causing a conflict with the assignment of the hostname to the NCPi .
Also, just for reference, it seems like the rebind protection wasn’t playing a factor at all. Both pre-fix and post-fix having it on or off seems to make no difference.
Anyway, thanks again.