Hi Cécile,
I just created a new landingpage and checked out the feature with the mobile version.
When I wanted to change to the mobile view within the editor of the landingpage for the first time, the system told me that the landingpage had to be updated in order to view the mobile version. I did that and then I could see that all texts and forms did adapt their width to the size of the mobile device. The only thing I could do within the mobile landingpage editor was to reorder the various assets like text fields, forms, pictures, etc.
I also tested the page on my iPhone and it really adapted concerning the width. So I did not have to scroll vom left to right, just from up and down.
Maybe you have to delete the mobile version once (now after the updated of Marketo) and then activate it again.
Best regards,
Katrin