Validation failed error in admin panel for Complex Page Collections

The code works correctly, but when you try to save through the admin panel, a validation error occurs. How to write it down correctly?

<details>
<summary>header</summary>

content:
items:

• '@self.modular'
• '@page.modular': '/action'
• '@page.modular': '/subscribe'
• '@page.modular': '/address'

</details>

What validator uses GRAV system? So I can fix the code. The site works well and I can't find the error with online validators. The function of editing pages through the admin panel is important for the site owner. All updates are installed.
P. S. There was still an attempt to make an automatic fix code in the new version of the panel. Yes it corrects, gives save, but change the page template on the Blog instead of Modular. And when you trying to save again writes the same validation error.