We found the issue. It seems to be a bug with creating ad-hoc releases. If you create an ad-hoc release which has processes, then BP does not seem to preserve the published status of the process when importing into another environment. To get around this issue, we ended up created a Release package first (via Releases -> Package Overview -> New Package) and adding all processes to the package. Then we create a new release from that package. This way, the Published status is preserved when importing the release into another environment.
Hope this helps