Metal Roofing Screws Breaking

Hello I was wondering if anyone can give me some insight as to why this is happening?
In October of 2018 I put on an exposed fastener metal roof. I was inspecting the roof yesterday and found a couple loose screws. I was able to unscrew them by hand only to find that the screw we’re actually broken. They were broke at various lengths anywhere from 1/4” to 3/4”from the neoprene washer down the thread. It was a clean break. The substrate of the roof is original 5/4” decking in widths from 8 to 10”. I then put 7/16” zip system sheathing over that. The main reason for the sheathing was to creat a flat deck surface as well as seal the surface due to the 5/4 having gaps in it up to an inch. Meaning the 5/4 decking was never installed with the boards butting against each other. On top of the zip system I put gaf tiger paw underlayment as a second form of protection. The screws used were #12 1 1/2” wood grip screws. I have checked and the certainly aren’t over tightened nor are they under tightened. The neoprene washers seem to be compressed nicely but not protruding outside the washer.

Why did this happen?
Could it be faulty fasteners?
What is the repair?

Could be from the movement of the framing/decking. If it tugs on the screw laterally enough, it can break them.

Sounds like the screws were of questionable quality too.

The fix is usually to remove what you can of the screw and go with a larger screw.