The more usual reason why devices would be making an IPv4 request despite having an IPv6-supporting connection is precisely because the server doesn't support I

The more usual reason why devices would be making an IPv4 request despite having an IPv6-supporting connection is precisely because the server doesn't support IPv6 - either the network it's on doesn't support it, or it's not configured for it, or more rarely the software on the other end doesn't have IPv6 support. The request can't just be translated into an IPv6 one because there's no way of knowing that the server even understands that, let alone what its associated IPv6 address is.