"I am looking into this - but as it seems to have gone away again - is
there a simple way to get that code back, and/or see what the diff is,
before/badrun/after?"

Maybe it's just a flacky test. It's hard to guess.

You can get the Git revision of a build by looking at details:

https://buildbot.python.org/all/#/builders/10/builds/2389
=> Properties: "got_revision: 8702b67dad62a9084f6c1823dce10653743667c8"

See also "Changes" tab.

Sometimes when a bug disappears, I just close it as "outdated" after a few weeks.