A couple Fedora releases back, I noticed that my laptop stopped using IPv6 to access dual-hosted services. I gave RFC-6724 a read, but it was much too involved for my small mind. Fortunately, it contained a simplified explanation:
Another effect of the default policy table is to prefer communication using IPv6 addresses to communication using IPv4 addresses, if matching source addresses are available.
My IPv6 is NAT-ed, so the laptop sees an RFC-4193 address fc00::/7. This does not match the globally assigned address of the external service. Therefore, a matching source address is not available, and things develop from there.
For now, I forced RFC-3484 with gai.conf. Basically, reverted to Fedora 26 behavior.