This is an error that took us an hour to figure out.
We are using the right-arrow icon as part of the content within a collapsible header to represent a state transition. (See image)
We found that the expand/collapse icon did not appear.
Further investigation reveals that if the header content contains a span with a class matching the regular expression k-i-arrow.* then the expand/collapse icon doesn't work. It could contain anything else, it was just that one class pattern that threw it.
See this example. https://dojo.telerik.com/EwAsaKAf/3
I also posted a forum message here: https://www.telerik.com/forums/expand-collapse-arrow-fails-to-appear-if-a-class-matching-a-certain-pattern-appears-anywhere-in-the-header
This is clearly a bug, and one that I suspect can be fixed easily.