Describe the bug

Purpose

Recently the test_numfig_disabled_warn test case has been flaky; although some of the intermittent failures have been noted for docutils HEAD, I've also noticed it occurring with other versions of docutils too.

Some example failure logs include:

• docutils HEAD: https://github.com/sphinx-doc/sphinx/actions/runs/18772174723/job/53603139421#step:10:4828
• docutils v0.20: https://github.com/sphinx-doc/sphinx/actions/runs/19662374866/job/56311219423#step:10:4879

cc @jfbu @haampie @AA-Turner

How to Reproduce

We are not yet certain how to replicate this problem locally. It does appear intermittently in our GitHub Actions continuous integration testing.

Environment Information

TBD

Sphinx extensions

N/A

Additional context

• Related to Problem testing #13996.
• Related to Tests: isolate a pair of HTML build 'numfig' test cases #14100 (flawed attempt at a fix).