I defined custom attributes for the embed tag but only for some class, for example like this:

[embed_article]
CustomAttributes[]=test

The custom tags are shown in the insert object window, but the entered values are not preserved when the object is published.
Upon re-editing of the object, the value for the custom attribute "test" is empty and in the template for the embed object the variable $test is not present.

NOTE: This is specific for this case when custom attributes are defined per class, custom attributes for the regular [embed] section work OK.

Steps to reproduce

1) Add this to content.ini.append.php:

[embed_article]
CustomAttributes[]=test

and clear the cache.

2) Create an object of class article and publish it - "object A"

3) Create another object of class article and publish it - "object B"

4) Edit "object A" and in it's text insert the "object B". The field for custom attribute "test" will be displayed. Enter some text in this field, and publish the object.

5) Re-edit "object A", go to the inserted "object B" and open it's properties window.

After that the field for the "test" attribute is empty while it should contain the previously entered value.