Missing .so files for Lambdas

I’m having an issue with deployment of a few of my Python lambdas. AWS is complaining that it can’t find relevant shared object files (.so) e.g. musl, libffi - we’ve tried turning off “slim” and also specifying the .so files in the DockerExtraFiles parameter, but none of these seem to work - the files don’t get copied and aren’t appearing in the bundle. Has anyone else experienced this, and is there anything obvious we may be doing wrong? These lambdas all work absolutely fine locally.