When you link or unlink task then certain conditions cause the sensitivity to
not reflect the actual state of links of the tasks.

To reproduce do this;
Open New,
Add Two Tasks,
Save (e.g. to file named tiny),
exit Planner,
Open (e.g tiny.mrproject),
Link the two tasks,
Notice how the Unlink button doesn't reflect theactual state of links.

To see the Unlink not clearing - ctrl -click where you were above until the 
UnLink button activates and then click Unlink. Notice how the Unlink keeps
active (sensitive = 1 ).