It seems that the icons in Confluence are currently rendered using fontset. This can be an issue for organization especially banks that have strict security constraint (fontset cannot be downloaded) as a result this will not render on customer instance.
I would recommend that we change the current method of icons being rendered using fontset being changed to HTML entities instead.
There is also a security issue which have been reported by Microsoft recently the link can be found below : https://support.microsoft.com/en-gb/kb/3079904
You can see an example of a customer instance not rendering the fontset icons as per the screenshot attached below : !fontset not rendering.jpg|thumbnail!