I don't think 60 fps is supported in HDR10+. I would say that it doesn't work for 4K/60 for the same reason that Extended Dynamic Range Video is only available on the iPhone at 24fps. 60fps wouldn't give each frame enough time to capture the range of colors and exposures that it needs to. It could also be the case that the phone can't stabilize and process that amount of data it in real time...
