Itsuki relationship is progressing differently compared to the others. Itsuki and Fuutarou are not skipping any stages in a relationship. Fuutarou's and Itsuki's development regarding the sisters for Fuutarou and Fuutarou for the sisters are tied together. They are both essential to each other to grow up and we have seen this in every arc.
The reason why fans get upset about that is that Itsuki has yet to interact with Fuutarou on an affectionate level compared to the others. That's why we get posts like "she is irrelevant" or "Itsuki hasn't done anything". This means there hasn't yet anything to happen on a romantic front in their interactions, but that doesn't mean that Itsuki doesn't like him or hasn't fallen for him. They have interacted a lot, and Itsuki has been actively trying to figure out "who is Fuutarou".
Even early on in the chapters, Itsuki is regarded as the serious type with a stubborn personality, which means she is often not honest with what she wants. I'd see Itsuki going as far as to completely hide her feelings for Fuutarou because she feels often intimidated by her sisters since she's the youngest and because in Itsuki's words "she doesn't want to get in his way" Compared to all the other sisters (save Yotsuba maybe), Itsuki would completely mask her feelings towards Fuutaarou, since I'd only be a bother to him, which is currently happening thanks to Ichika. And apart from her interactions, we have seen in the last arc, that it's implied that she's keeping secrets she wouldn't even tell her sisters.
So, imaging Itsuki running up to Fuutarou in order to kiss him (on the lips or cheek), as a desire to have Fuutarou stay together with them forever. Fuutarou would never be able to figure out, which sister it was. Hell, the whole kiss might have even been a complete accident.