When Emphasis (for example Bold) is applied to the text inside custom style and then Content is saved and reopened for edition, previously added Emphasis cannot be removed.

Steps to reproduce
1. Clean installation of eZ Platform 2.5.5
2. Create a custom style:

ezrichtext:
    custom_styles:
        test_block:
            template: 'styles/test_block.html.twig'
            inline: false

with template:

<div>{% apply spaceless %}{{ content|raw }}{% endapply %}</div>

3. Create a new Article. In Intro field apply test_block Custom Style to some text. Highlite single word from inside of the test_block and apply Bold to it. Publish Article.
4. Edit article. Highlight previously Boldened word ant click on Bold button on the tooltip.

Result
Bold Emphasis is not removed from highlighted word.

Expected Result
Bold Emphasis is removed from highlighted word.