It appears that the timer task priority is set lower than osPriorityNormal (24), so the timer task must wait for task A to finish Attitude_Calculation() – a 5ms operation. Only then can task B wake up, even though it has the highest priority of all.
As Richard suggests, using direct-to-task notification would resolve all of this, and your synchronization latency will end up even shorter than if you increase the priority of the timer task and stay with event groups.