amazon web services – Managed platform update failing with Incorrect application version error

I have 6 Elastic Beanstalk Python 3.7 environments running the same application. Over the course of the weekend, 5 of them have successfully, automatically auto-updated to the latest platform update:

Platform update from 64bit Amazon Linux 2 running Python 3.7 3.3.4 to 64bit Amazon Linux 2 running Python 3.7 3.3.5

Auto update has been running successfully all year but, today, one is steadfastly refusing to upgrade and is just reporting:

FAILED – RollbackSuccessful
Successful abort of the Managed Action.

It appears to deploy the new instance successfully and both the new and old instances get to a point where they say OK
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However, instead of the old instance being shut down, after a few minutes, the new instance suddenly reports:

Incorrect application version “app-bd432-13735_111407” (deployment
280). Expected version “app-bd432-13735_111407” (deployment 277).

and the upgrade is rolled back.

So far I have tried:

  1. Just retrying the upgrade
  2. Redeploying the latest version of the app, then retrying the upgrade
  3. Performing Rebuild environment before trying the upgrade

But the result is the same. I can’t see any indication in the logs of either instance (old, surviving or new, failing) that offers any help but will be happy to add log info if anyone can suggest a specific log to extract.