Sorry, did not get around to replying sooner.
I have looked at the links you provided and there appears to be some overlap. No solution yet though.
About steps to reproduce. I start with a new canned response. And in the editor I type some text, select it and choose insert link. In the insert link dialog, I choose End-user Ticket Link and then insert.
When I select the link, it shows a small dialog with the expected link. Hopefully the attached screenshot shows this.
Next, I select add response and it gets added to the list of canned responses. When I open it and select the link again, the {} has been escaped by %7B and %7D, as the other screenshot hopefully shows.
I am doing this in Firefox on Ubuntu 18.04.