This issue is related to how the external calendar displays the .ics file. We can either send it over with HTML or without - however, without HTML, it would be extremely difficult to read the file.
While these files simply reflect normal HTML, it appears that some external calendars may not display it correctly. However, iCal does seem to display the HTML in code rather than display the intended appearance of the note.
At this time, this behavior appears to be a limitation on some external calendars at this time. If you have any further questions, please feel fill out the form at www.greenhouse.io/asksupport.