When a transition instance is cancelled in a scope and the execution tree structure within that scope changes in the same transaction before the cancellation is executed, then the transition instance cannot be cancelled.

Examples:

• sub process with a single active transition instance; reproduce steps: in one modification, start another activity and cancel the transition instance; cancelling the transition instance fails
• sub process with two two active transition instances; reproduce steps: cancel both transition instances; cancelling the second fails

The reason is compaction of the execution tree when there is only one active path of execution in a scope and expansion when there is more than one. Since we use the execution id as the transition instance id, the transition instance id after expansion/compaction may not be valid anymore since another execution may be responsible for executing the async continuation then.