Currently you can enable mode 3 without setting a bootstrap address. At first it will work, as it uses regular DNS to create the TRR connection, then it enters strict mode. However, if the connection drops, it can't resolve the hostname, so it fails completely.
We should make sure that even in mode 3, the connection to the DoH server bypasses TRR.
Also some people set the trr url to have an IP literal. For example https://18.104.22.168/dns-query
I'm not sure if we have a bug for this, but it seems that sometimes the first resolve using TRR fails. We should add a test for that.