This weekend, I upgrade our Jira instance from 5.0.6 to 5.2.6 and the Jira Workflow Toolbox plugin along with it from 2.0.11 to 2.0.20.
In our workflows we use the "Validation on linked issue" validator and in one of the validators, we have the option set Linked issues belong to: "any but current project". This used to work correctly.
However since the upgrade the validator is always rejecting the issue transitions, even though on our tickets the linked issue is in a different project. If I alter the validator option to "any project" then the validator permits the transition.