Had the same issue ever since I dropped my phone on the wooden floor, it fell directly on its screen. The distance was 140-160 cm, I believe. No other signs of damage whatsoever, apart from a small whiteish stain on the bottom of the screen near the home button which is only (and barely) visible on maximum brightness when the display is completely black. I thought Taptic Engine was affected at that time, but all other sorts of vibration seemed to work how they’re supposed to. I’d say that this IS a hardware issue after all, however I have to admit that just like for many others, for me this happens pretty randomly, it can occur once in 5 clicks, can not occur in like 40 clicks or can continuously repeat for 10-15 clicks in a row and so on. I really think this is some perfectionists’ thread since most would never even notice something like this. Let’s just call it “custom button feedback” :D