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If you don't specify a user here, the transition on the related issue is performed as the same user who triggered this post function on the origin issue by default. Therefore that user must have the necessary permissions on the related issue. In some setups, that user might not have the required permissions on the related issue or even access to the project of the related issue.
In combination with the User is in Any Users condition, you can hide a transition from all users other than the 'Perform As User' user.