To be able to investigate the reported issue, It would be useful for us to know these parameters for one specific mission where you observed the speed selection not to be honoured:
Note that depending on the overlap and the altitude you select when planning your mission, the maximum speed your drone can reach will change and so will the minimum speed. Since the camera of your drone can capture one image only every N seconds, we have to make sure that your drone will not fly too fast for guaranteeing the expected distance between images for the selected overlap and altitude.
High, medium and slow speed don’t have to be considered as absolute values. The higher will be the calculated maximum speed, the higher will be the minimum speed the drone will obey when you select it.
e.g. with P3P:
50m and 80% overlap, fast: 6.5 m/s
50m and 80% overlap, slow: 4.9 m/s
100m and 80% overlap, fast: 10 m/s
100m and 80% slow, slow: 6.5 m/s
50m and 70% overlap, fast: 9.8 m/s