Flash state in metadata shows wrong

I'm sorting through photos I've taken of my girlfriend. Made a folder for only pictures taken with a flash. But when I sort the pics by Did Fire and Did Not Fire, the metadata has it incorrect. Some pics I know I used flash on are marked as Did Not Fire, and vice versa with some pics I know I didn't use flash on.

I would have thought it was the camera writing wrong metadata, if this hadn't been the case with pics from three (3) different cameras. Two Canons and one Fujifilm.

So what's up, is this a widely-publicised problem, or what?