I tried to reproduce the problem but to no avail. Here is the button that I used in the content page and as you can see in the attached screenshot, it is rendered fine.
| ImageUrl="http://www.ewbi.com/ewbi.develop/text/text.ashx?text=%22dynamically created text%22&bold=true" />
Please open a support ticket and send us a sample project where this issue could be reproduced. We will check it and do our best to help.
the Telerik team