This issue is happening due to the insertion of the /n character in the comment editable area. This character breaks the logic of some content filters and they are replacing the greater than and less than characters in the HTML mode. 

Possible workaround is to disable the ConvertCharactersToEntities filter:

C# example:
RadEditor1.DisableFilter(EditorFilters.ConvertCharactersToEntities);

VB example:
RadEditor1.DisableFilter(EditorFilters.ConvertCharactersToEntities)