I have an idea as to why this happened. I’m 90% sure this is it:
Is your Araquinid totem? If it is, that explains it. Totem Pokémon originate in US/UM, so they cannot be traded to S/M.
Usually hacked Pokémon can bypass the hack check and trade if you’re using link trade. The reason why you can’t trade them on GTS is because most of the time hacked legendaries or mythicals are in cherish balls, and you can’t trade event Pokémon on GTS. I think the real reason is that your Solgaleo was holding either the Solgalium z or the Ultranecroium Z