Interesting/mysterious indeed. Since dig works fine, using the very same upstream DNS providers, I’m not sure whether an ISP issue can be the explanation.
I’m more thinking about an issue with some depth of Raspbian system files/libraries used by curl and wget for their DNS resolution. And since my Raspberry Pi Zero W does not have this issue (and a few thousand others neither), it would then only occur with some very specific network behaviour/setup.
Could some tcpdump be helpful to which packets are going where/coming from where when doing the curl/wget commands?