Highlighted text on Books not syncing

I use the Books app in several devices, mainly iPad and iPhone.

The books I’m reading and the page where I leave it get synced between them.

However, the highlights in the text I apply on my iPad does not appear / does not get synced in my iPhone (even if, when I open the book on the iPhone, it takes me to the point where I left it on the iPad).

(iCloud and syncing settings for Books are on).

Any idea why this is happening? Why are the highlights applied on my iPad not appearing on my iPhone? Any idea about how to solve it?

Thanks