2018 Quarter Weighing 5.1 Grams

So far its looking like you've got a rolled-thin error instead of a wrong stock error. A coin struck on a planchet punched out of stock that was rolled too thin on the rolling mill is called a rolled thin planchet but as an example, a 1974 quarter was struck on a rolled-thin planchet. It weighs 4.92 grams instead of the normal 5.67 grams (non silver normal weight 5.67).
https://www.coinworld.com/news/us-c...ting-wr.html

You can also check if there is any differences you can see when held next to another 2018 with a normal weight, any tapering/thinning.