Iansui subsetting is not working properly

[aaronbell]

Describe the bug

As reported by @ButTaiwan here, it appears that the Google Fonts subsetter is stripping out crucial glyphs for Taiwanese Hokkien.

To Reproduce

See this codepen

(Ignore the issues with the accented glyphs as that is unrelated to this bug report). There are many Chinese characters for which we see fallback and appear not to render. However, I can verify that these glyphs are present in the font and render as expected on the GF website as well as in the font source / binary.

From the codepen:

From GF website:

[ButTaiwan]

It looks like there is same problem in combining mark symbols too.

Google fonts subsetter puts U+0301, U+0300, U+0302 and U+0304 correctly, but U+030C, U+030D, U+0306 and U+0358 are missing.

https://codepen.io/ButTaiwan/pen/YPzQBQg

Using Iansui installed in PC: