The syntax of text-underline-position is auto | [ under || [ left | right ] ] which means auto should never be combined with other values.

However, in Appendix B: Default UA Stylesheet, we have

:root:lang(ja), [lang|=ja],
:root:lang(mn), [lang|=mn],
:root:lang(ko), [lang|=ko] { text-underline-position: auto right; }
:root:lang(zh), [lang|=zh] { text-underline-position: auto left;  }

which doesn't match the syntax above.

One of them should be updated.