How to reproduce

• Edit a content
• Edit a paragrah
• Apply the style important to this paragrah => the text is framed on blue background
• Change the style of the paragrah to secondary'

Actual behavior

• The style important is not removed
• The two styles (important and secondary) are applied
• If you change the style a third time, the paragrah will have the three combined styles

<p class="important introduction secondary">Lorem ipsum sit amet</p>

Excepted behavior

• The paragrah style is removed to each changes
• Only one style is applied on a paragrah

Note
The only way to change the style is to back to the normal style, then choose the other style