Shutter Speed on Lumix G85

I have trouble understanding how to set a fix shutter speed with my camera.

Once I chose a SP, my camera take the picture with the right one then if I take another one shortly after it tries to adapt the shutter speed to have more or less exposure.

Is there a way to remove this option?

For info, I tried in S and M mode, with manual ISO, manual focus, manual WB, and I made sure that the "multiple exposure" option was off in the setting menu.