It is intermittent, though not that rare. Might happen once every day or two. Maybe a bit more. But sometimes closing the tab and reopening it doesn’t help, so maybe it’s from some kind of race condition on page load.
As I’ve stated before, this is much slower than double tapping. Besides, when double tap to highlight works everywhere else, it’s reasonable to get into the habit of doing so. Meaning that I’m going to try to do so out of habit on Discourse as well, which makes it that much more jarring when it doesn’t work as expected.