I’ve just noticed on my iPad Mini if I let the battery go right down to 10%, when I plug in to charge from the iMac I hear a beeping sound, and when looking in iTunes the device is not shown mounted (and yes, I’ve turned off USB Accessories). If I then open the home screen, suddenly the device mounts, beeping stops and charge indicator shows.
I usually don’t let the iPad go that low, about 30 or 40%, and when connecting have never observed that sort of behaviour before.
Does your issue occur when plugging in at 80% as well as say 10%, or is it like I believe mine is now, only occurring at 10% or below, it’s not an answer but you might not be alone with this one.