Board Thread:Manga/@comment-38849221-20190321163015/@comment-37493549-20190322020229

It seems like a plot hole for now. Zeke implied something bad would happen if they touched, that's all we know.

Also I think Eren can't simply touch a person with royal blood to trigger the founding titan, they have to be in titan form. Otherwise he could just do it with Historia. Zeke transforming into his titan would give away their position.