There are two things:
1. Placeholder data for another Lycanroc hidden in Sun and Moon games discovered by the hackers.
2. The fact that GLadion has Midnight form and Olivia has Midday form. Ash's Rockruff evolving into either of the two would feel...off, because the forms have already debuted and are both owned by major characters. So Ash getting Midday or Midnight would feel unnecessary. This is why fans think the placeholder data could indicate Ash gets a new Lycanroc form.
Yes those are the only 2 reasons why I think a 3rd form exist.
Ash has had Pokemon already owned by major characters before. He had Torterra and Gliscor which Paul had, before he caught Krokorok Clay had one, Greninja which the ninja guys had and yes I know Ash's was a special one but still they were recurring characters. Just because 2 characters have a Lycanroc with a different form each doesn't mean Ash isn't getting one of them. I mean yeah its still possible for Ash to get the fabled 3rd form if it happens, but right now I see Midday form being the form to get.
That's not necessarily it, like I couldn't give a crap about Gladion's Midnight form Lycnaroc, but they PURPOSEFULLY changed Olivia's Midnight form to Midday, that literally makes NO sense because if Ash's was going to have his Rockruff evolve INTO one relatively soon, and potentially have a rematch with Olivia, because he hypothetically loses, its NOT going to make ANY sense to have a Midday vs Midday Lycanroc especially since neither are shiny (otherwise Rockruff would either be Blue or Lycanroc would be blue as well)
And even then if Ash somehow beats Olivia, its still not going to make any sense, because it would've been UNNECESSARY to change Olivia's Lycanroc, given that Ash's was going to just hypothetically get his relatively soon after.
The only thing that could resolve the big confusion mess in my head is if it evolves into 3rd form, otherwise I'm going to be pissed if it evolves into Midday form, and changing Olivia's Lycanroc for no DAMN reason. I'll even be more pissed if Ash DOES lose to Olivia, and brings back Midday Lycanroc vs Midday Lycanroc, because that will definitely make me ask "WHY THE HELL DID YOU CHANGE HER LYCANROC?" So if Ash does beat Olivia, and then evolves into Midday, at least I'll be less pissed, but still pissed that they needlessly changed her Lycanroc because "Wynaut"
Only way the writers/producers get away from being completely stupid (outside of Ash actually beating Olivia and not using his Lycanroc against her's, unless Ash DOES get Midnight) is if it evolves into the hypothetical 3rd form.