fetch(8) currently picks one IP address to try and fetch a HTTP(S) resource from, and fails if the TCP connection cannot be established.
fetch(8) should implement Happy Eyeballs https://datatracker.ietf.org/doc/html/rfc6555 and try to establish connections to some or all of the IP addresses a name resolves to, and use the "best" connection to download the resource. This would improve the reliability on dual-stack systems, where sometimes, certain addresses cannot be reached, due to a variety of reasons.
See e.g. https://marc.info/?l=openbsd-tech&m=157475113130337&w=2 . I have not looked to see if they incorporated this patch into their tree.
Note RFC6555 has been obsoleted by RFC8305
(In reply to Mark Linimon from comment #1)
I think that thread/patch is talking about OpenBSDs resolver unwind(8), not fetch/libfetch.