Steps to reproduce

1.  Create the following decoration to the dam-app (<LIGHT_MODULE>/decorations/dam-assets-app/apps/dam-app-core/dam.subapps.jcrdetail.yaml)

   form: !override
     properties:
       test:
         $type: textField
   

Expected results

All properties/nodes bellow form are ignored and only the new field test is added to the definition

Actual results

Properties/nodes bellow form are not ignored but merged, so original fields + new field are displayed.

Workaround

Use this decoration instead: (<LIGHT_MODULE>/decorations/dam-assets-app/apps/dam-app-core/dam.subapps.jcrdetail.form.yaml)

  properties: !override
    test:
      $type: textField

Or change the format of the decorated definition from array of properties to map of properties.

Development notes

The !override directive behaviour is not homogeneous and difficult to understand why.

The is a note in the documentation:

Due to the syntax of a YAML list, you cannot apply !override to a list item. To bypass this constraint, use a YAML map instead of a list to apply !override to an item. Even if the definition class expects a list, the YAML file can define the item with a map.

Roman's suspicion is "Seems to be a different constraint (meaning you cannot override a field written as an array item) but nothing about overriding whole array"