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I mean, they’re fogging because the top of the mask leaks, and the breath takes the path of least resistance and goes around rather than out of the mask.

Yup, exactly. I use my glasses to seal the top. It's weird but it works.

Likely, but not necessarily. My silicone half-face (with disabled check valves) slightly fogs my glasses because enough water vapor is leaving the filter close to my glasses. I'm sure it's not leaking, because I can feel the silicone pull away from my face when it does.

For me, fogging my glasses means I have a leak and the air is being forced out gaps around my nose. Maybe under extreme weather conditions a perfect fit would still fog, but that's not what the op is talking about.

I think it's more about the direction of the water vapor than the fact that the masks can pass vapor. If there was no leakage one might expect vapor to leave through the mask uniformly or in the same pattern as it did without masking (e.g. mostly forward.)