bounce processing broken

Since upgrading to 4.2.2 (from 4.1.3) we get a lot of these:

Failed Processing: +kU1P4XK2YUuQGnfl- (+MKgw6TD8-), Action: b, Job ID: 126532, Queue ID: 2675337, Hash: 327414d86132c01e. Reason: Mandatory key(s) missing from params array: body

I've attached the (slightly redacted to remove the recipient's identifying information) e-mail that triggered this particular failure. We have tons of examples, if you need more. This example is a relatively straightforward bounce.

The error reported is always "Mandatory key(s) missing from params array: body".

Thanks,
Ward.