I can confirm this is an issue. Experienced the same on v19.07.2 when I needed to re-install luci-app-https-dns-proxy. I got back the DoH menu by "upgrading" to v19.07.2 over the same v19.07.2, while keeping the settings, then manually restoring all my packages, including luci-app-https-dns-proxy.
Later on I tried upgrading to the new v19.07.3 on another router without keeping settings, then installed DoH from scratch. No DoH menu. That suggests there might be an issue with some recent versions of luci, but I couldn't find anything suspicious in dmesg and logread output.