You have eZ Platform project with multiple siteaccess, which runs on Platform.sh:

• en.site.com (en siteaccess) (default siteaccess)
• fr.site.com (fr siteaccess)

When you create a stage (or any other) environment for this project on Platform.sh, following URLs will be assigned to created env:

• en—site—com—%ENV%%REGION%%PLATFORM_PROJECT_ID%.us.platform.sh
  *fr—site—com—%ENV%%REGION%%PLATFORM_PROJECT_ID%.us.platform.sh
  And those URLs will not be mapped to any siteaccess, so default one will be used for each of that stage env URLs. It means fr—site—com—%ENV%%REGION%%PLATFORM_PROJECT_ID%.us.platform.sh will open “en” (but not “fr”) siteaccess.

You can manually modify stage env siteaccess match parameters. Which might be a huge pain if there are tons of siteaccesses in your project.

Also, you can install https://packagist.org/packages/contextualcode/platformsh-siteaccess-matcher-bundle