Stencil Buffer Back Cap rendered incorrectly

Dear experts,

I've encountered a strange problem when doing depth-fail stencil shadows. The back cap seems to be interfering with the stencils causing them to marked as lit rather than shadowed.

I'm using a 2-way stencil btw. I'd just like to ask for some first-glance indications. I'm not asking to be spoonfed with a solution.
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