When I import video files from my iPhone 7, LR fails to import the GPS co-ordinates for many of them. It does import the co-ordinates for *some* of them. A simple ExifTool command easily extracts the co-ordinates for all the files, regardless of whether LR failed or succeeded. There does not seem to be any obvious difference between the files that succeeded and the ones that failed.
I am using LR 6.10.1 on Mac Sierra 10.12.5 and my iPhone is running 10.3.2.