Steps to reproduce:

• Add css config similar to https://doc.ibexa.co/en/latest/content_management/rich_text/extend_online_editor/#custom-css-classes

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

• run php bin/console c:c
• Edit Richtext block and set classes special and tip_box for a paragraph
• Publish Content
• Edit config/packages/app_custom_class.yaml and remove special from choices
• run php bin/console c:c
• Edit previously changed Richtext block and check the paragraph's classes

Result:
Classes special and tip_box are both gone

Expected Result:
Class tip_box should still be set