How to reproduce

To reproduce this issue just copy config from this doc page:

https://doc.ibexa.co/en/latest/content_management/rich_text/extend_online_editor/#custom-css-classes

and change multiple flag to true:

ibexa:
    system:
        admin_group:
            fieldtypes:
                ezrichtext:                            
                    classes:
                        paragraph:
                            choices: [regular, special, tip_box, warning_box]
                            default_value: regular
                            required: false
                            multiple: true 

Bug description

When I select multiple classes for the paragraph in the richtext then those classes are saved and rendered with commas.

<div class="ezrichtext-field"><p class="regular,special,tip_box,warning_box">test</p>
</div> 

Also, in richtext custom attributes widget, when I want to manage classes and I click again on the class that I've selected then nothing happens. So to me it looks like there's way to unselect given class.

The only way to change selected classes is to remove the custom attributes completely from the paragraph by clicking "Remove" in the widget and then adding custom classes again from scratch.