Adding custom tag inside RichText will be saved correctly on create, but all attempts on editing values of the said custom tag will fail (changes to the custom tag won't be stored).

Step to reproduce
1. Clean installation of eZ Platform 2.4
2. Add testct custom tag to ezplatform.yml:

ezrichtext:
    custom_tags:
        testct:
            template: 'AppBundle:custom_tag:test.html.twig'
            icon: '/bundles/ezplatformadminui/img/ez-icons.svg#error'
            attributes:
                some_text:
                    type: 'string'
                    required: true

and enable it for RichText:

ezpublish:
    #...
    system:
        default:
            fieldtypes:
                ezrichtext:
                    custom_tags: [testct]

And create AppBundle:custom_tag:test.html.twig template file containing:

{{ params.some_text }}

3. Create a new Article. In said article in Intro field add testct custom tag with value: Test 1.
4. Publish Article
5. Edit same Article
6. Edit testct custom tag inside Intro field. Change its value to Test 2.
7. Press Publish

Result
In the new version of Article custom tag testct inside Body field will still have a value of Test 1.