To summarize the issue: DNS is done optimistically over UDP because it's faster, but this doesn't work when DNS responses are large because of the design of UDP. TCP should be used as a fallback mechanism when responses are large. This is uncommon normally, but increasingly DNS responses are large in special scenarios; for instance when you're querying an internal DNS for service discovery (read: k8s or nomad deployments, most commonly).
Musl's maintainer interpreted the spec for a libc's resolver to not require TCP fallback (source: https://twitter.com/RichFelker/status/994629795551031296?lan...), so for a long time Musl simply didn't support this feature, justifying it as better UX because of the more predictable performance.
I don't agree with the maintainer on this interpretation, but I am glad the feature was added and the issue is no longer a concern as an otherwise very happy Alpine user!