I suspect smartdns hasn't redirected properly. I ran into similar issues with AdGuardHome.
The best way to resolve it is to move dnsmasq and make it an internal resolver only (bumping it to port 5353). Then have smart dns be your primary resolver. Another reason for moving dnsmasq is because you dont want additional hops in your dns query chain. Also dnsmasq forks on every request and uses additional memory.
With AGH you can set your moved dnsmasq instance as a PTR or downstream lookup, and use DOH for upstream requests.
Additionally you have to understand that OpenWrt has two sets of DNS.
Its upstream that is derived from your ISP DHCP. The router by default will use this unless you change it manually.
And the DNS it hands out. Normally this is dnsmasq and uses the same upstream the router does.
However as you have got smartdns, you are bypassing the ISP dns and router dns for a direct lookup from your new upstream server. Thus you must make sure your downstream clients are using the correct DNS. This you set by using dhcp option 6 to pass it to your clients. In theory it should just take over but really you should be specifying it as part of a good network.
I have a more detailed explanation on that thread. The same principles apply to smartdns. Just replace AGH with smartdns.
(edit) - to check it properly redirected dnsmasq? look in the etc/config/dhcp file and find the port line.