I tried again, created the same custom field, and now it appears on the page editor. It seems that the custom field was not saved in the first instance.
So far, it works!
I also have a problem with the appearance of custom fields on the page editor.
For a demo purpose, I’ve created a simple custom field, like I normally do with ACF or Metabox. The issue is it does not appear on the page editor. I used a multisite for this purpose. But I tried the same on a single website, and it shows as expected. Where is the problem? I understood this plugin is multisite compatible. I’ve sent a ticket as well.