MATHEMATICAL MONOSPACE DIGIT ZERO (`U+1D7F6`) is unaffected by the `zero` OpenType font feature.

[jmcwilliams403]

I believe this may have something to do with how Mathematical Monospace characters are handled as deriving variants for Quasi-Proportional builds.
Mathematical Monospace Digit Zero:

Compared to Mathematical Sans-Serif Digit Zero:

This has also been an issue on released fonts too:

<p style="font-family:'Iosevka Extended';font-feature-settings:'ss11','zero'1;">
⏨<br>
⑩⑳⑽⒇⒑⒛⓪⓴⓾⓿<br>
❿➉➓<br>㉚㊵㊿<br>
𝟎𝟢𝟬𝟶
</p>

Outside of some other zero-derived characters whose slash is forced on or off by some other means (𝟘, 🄍, 🯰, etc.), this is the only problematic one.