Quick answer: On video from behind, the putter face doesn't point at the chosen line at address; the ball starts off the read line within the first meter.
What it does to the ball
The face decides almost all of a putt's start direction: one degree open is several centimeters of miss at 3 meters — well-read putts, missed.
Root causes (by frequency)
Not using the line on the ball or an intermediate reference to aim
Setting up with the body first and turning the face afterwards
Eyes off the vertical over the ball, distorting the perception of the line
How to fix it
Draw a line on the ball and aim it down your read line; set the face square to it and, only then, the body
Drills that fix it
The tee gate on the green · 10 min · 20 reps
Start the ball off the face on your chosen line Push two tees into the green forming a gate barely wider than the ball, 30 cm in front of the ball on your line. Hit 2 m putts rolling the ball through the gate without touching the tees. Eight out of ten to end the session.
Chalk line on the green · 15 min · 20 reps
Align the face to an objective reference Snap a straight 2 m chalk line to the center of the hole, or lay down an alignment stick. Draw a line on the ball too and set it on the chalk line. Square the face to the chalk and hit putts, checking that the ball's line rolls end over end without wobbling.
Frequently asked questions
What is "Putter face misaligned at address" in the golf swing?
On video from behind, the putter face doesn't point at the chosen line at address; the ball starts off the read line within the first meter.
What does putter face misaligned at address do to the ball?
The face decides almost all of a putt's start direction: one degree open is several centimeters of miss at 3 meters — well-read putts, missed.
How do you fix putter face misaligned at address?
Draw a line on the ball and aim it down your read line; set the face square to it and, only then, the body
Sources
Dave Pelz — Dave Pelz's Short Game Bible
Dave Stockton (con Matthew Rudy) — Unconscious Putting