Only when applying the formatting for the second time it does work.
The same occurs with Underline and Italic formatting. When doing this on a
specifiek word in the line the problem does not occur.
The problem might have something to do with this issue:
https://dev.ckeditor.com/ticket/6706
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Ticket URL: <http://dev.ckeditor.com/ticket/9013>
CKEditor <http://ckeditor.com/>
The text editor for the Internet
* keywords: => IE9
* status: new => confirmed
* version: => 3.6.3
Comment:
It's a recent regression, reproducible only in IE9 from CKE 3.6.3 rev
[7366]
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Ticket URL: <http://dev.ckeditor.com/ticket/9013#comment:1>