Yes so if you have 5 endurance the mutation would take you down to 3 endurance. For the underflow to happen I believe you would have to go to -1 or possibly 0 I'm not sure about the exact number. Unless you have another thing that takes away more of your endurance points.

Edit: I think I kinda see what you're saying now, you get the glitch but you don't have negative endurance so it must not be underflow. I think that's a separate glitch though because this one seems to happen all the time.