Answered
Masking bug: Masks don't work with lines.
The mask layer doesn't have the lines reveal the layer behind the mask for some reason. I put some pictures here to show the problem:

Can someone please tell me how to fix this?
The mask layer doesn't have the lines reveal the layer behind the mask for some reason. I put some pictures here to show the problem:

Can someone please tell me how to fix this?
This is not a bug.
Masks can only be fills.
Convert your line to a fill:

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