This is another manifestation of a bug that has long plagued us. I believe this fix should last for a while, as it worked in all the cases I tried:
https://github.com/discourse/discourse/commit/02f6db4e59e03f49739be7e228641613dab0ae26
(the commit message has more detail.)
I suspect this has been here since our last Ember upgrade at least. Probably longer.