Seems quite normal, the material used to make those lids is rather brittle, as such it most likely is shattering into many pieces, this is good as there is no flow restriction at all, compared to other softer burst discs that only rupture in the middle.
I say you are onto a good thing, stick with it!
Edit: Maybe this would be a good time to invest in Gatorade shares.
Find a similar material and punch out your discs, that is if you don't want to be drinking a lot of Gatorade!
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