There appears to have been a regression in the support for a native HTML checkbox's indeterminate IDL attribute, as recent testing has shown that it is returning the unchecked value (0) rather than the mixed value (2) when specified.
Test case: https://scottaohara.github.io/tests/attr-indeterminate/
Reviewing with the accessibility checker, the native checkboxes are returning a value of 0 when given the indeterminate IDL attribute.
I found these previous issues which reported on this bug, and were closed as fixed (hence assuming this is a regression). I am linking to them here in hopes that they might be somehow helpful in fixing this new instance of the mixed state not being properly exposed.