It seems to be not every time but pretty reliably. Interestingly, this time it did happen with Nightly, so I'll move over to Bug 1631814.
Bug 1629984 Comment 16 Edit History
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Correction - I was able to get it to happen with Nightly after all. (There may be some subleties of the process that I didn't reproduce before.). I've put the steps I did on Bug 1631814.