Collection of a specific page's modules

This is the problem:

I want to point to the modules of a specific page from the collection of another page. Looking at the documentation, this is achieved as follows:

content:
    items:
      '@page.modules': '/blocks/serveis'

I save from the admin plugin (expert mode) and it works perfectly, but if I switch to normal mode, the code changes automatically and stops working:

content:
    items:
      '@page.modules': /blocks/serveis

If I go back in and save from expert mode, the validation error pops up (invalid entry for Items), but it works again. It is very curious when I switch back to normal mode and save (without changes). Then the code changes again and stops working:

content:
    items: /blocks/serveis

I am aware of the sensitivity of the text in the yaml format, but I still think there is an inconsistency with the Admin plugin. Or… am I doing something wrong?