Some Builds lose build plans

When running arc land, some of the worker tasks fail because the DifferentialDiffExtractionEngine cannot find the buildPlan of a build.

The error is produced at$364 where $plan is null.

It might be linked to

Reproduction Instructions
It’s intermitant on our server, I wasn’t able to find a way to make it crash directly

Phabricator/Arcanist Version
arcanist 01e91dc260a737376701a4908af04b67ff2791cf (1 Jul 2020)

Sorry, I can’t help with a problem I can’t reproduce.