Detailed logs to file for parallel builds?

I recently switched my Packer configuration to build two amd64 and arm64 AMIs on AWS in parallel, which is very nice.

The one downside is I can’t seem to capture the build logs. The console is getting logs from both processes intertwined, any combination of stdout/stderr redirection with or without tee (with or without PACKER_LOG_PATH) to a file always ends up the same: the file during the build captures everything (I think), but after the builds are done, the log file is severely truncated.

Here’s my latest command:

  AWS_PROFILE=staging AWS_REGION=us-east-1 PACKER_LOG=1 \
    packer build -color=false . 2>&1 | tee .packer.log

Is there a proper way to do this, or is this just broken? Packer 1.10.3