Can't get seacret valiables only when run with PR triggers

Use ‘Bitrise Start Build’ to execute multiple workflows from a single trigger.
If PR is triggered and executed, it will fail because it cannot get the value set in the Bitrise Access Token.

access_token: <unset>

Succeeds when executed from Slack or manually.

Is this a bug? Or is the specification that triggering from PR does not work?

Thanks for your help.

Is your secret set to be exposed for pull requests (see the red switch under the value for the secret)?