Nothing to object @david.hewitt, but before making changes to our CLI/API Client we need to be able to reproduce the issue and understand the reason, to make sure that it won’t have any other implications for other users.
I’m also sure that the problem is not the migration itself… we just need something that reproduces the issue! 
Launching random migrations to random environments unfortunately works on my end, and with your exact configuration (Node 16, undici@5.16.0, @datocms/cli@1.0.22, @datocms/cma-client-node@1.2.3).
Btw, undici should already add an ‘accept-encoding’ header if it’s missing: