According to German Nintendo website ntower, players who have over 100 friends on their Switch and try to host a private multiplayer lobby in Mario Kart 8 Deluxe are greeted with a message that stops you from successfully hosting a race. The game requests that you add more friends by going back to the main Switch menu which ultimately stops you from hosting the private race with your friends.
This could be something to do with how it was ported, perhaps. On the Wii U you weren’t given the option to have over 100 contacts but with the Switch you can. Hopefully a patch will soon be on its way to remedy this glitch.