So I got tired of getting ghosted here and asked the question over on the US forum after it went live there.
The Community Manager, LeCron, came back with this answer from the development team within a few hours and gave this response:
It is intended that the button won't be visible sometimes. It happens when there's no better alternative for that particular task.
When exchanging the tasks, the game grabs from a pool of tasks. The task after the exchange should be easier than the previous one. If there is no easier task available in the pool OR the tasks exclude each other (for example, they cannot be of the same type), the button will not be available.
So there you have it, not a bug, investigation complete.