Adding flagellum decreasing the speed

Speeding unit decreasing the speed.

In the video i didnt pressed the return back buton which related another bug so i can show the this bug

I’m pretty sure that you just hit a situation where due to the shape of your cell the additional mass of the flagellum slows you down more than the thrust it adds. This is because flagellum thrust is dependent on the angle it ends up finally pointing once you go into the stage. For most effectiveness you want a thing and long cell with the flagellum at the back, that way the vector between them and a line going straight forward is as small as possible to ensure maximum thrust.

Based on that analysis, I’ll say there’s no “bug” here, unless someone can replicate the seen cell and check the math to see if something else is going on with the calculation.