Inconsistency in style formatting behaviour between first edition and coming to edition after save

Steps to reproduce:

• Create a brand new page with a text (first edition session) in WYSIWYG mode
• Apply a style to this text with CKEditor (Styles > Box)
• Then once again, apply a new style (Styles > Warning box for example)
• See how the style is updated for the text - same box around the text, but different colour, see the source:

(% class="box warningmessage" %)
(((
test text
))) 

• Save the page
• Come back to WYSIWYG edit mode (second edition session)
• Select the text - highlighting or cursor somewhere in the line
• Try to apply a new style (Styles > Success box for example)

Expected result: the new style is applied the same way it was applied on first edition session

Actual result: a new style is applied inside the existing one, see source:

(% class="box warningmessage" %)
(((
(% class="box successmessage" %)
(((
test text
)))
)))