The use of the excavator to mine asteroids increases the mass of the system
When it extracts ore from the asteroid to the ship, the total mass of the system cannot change, because it is moving the ore from the asteroid to the ship and not creating ore.
Apparently, the game is keeping the total mass of the asteroid fixed and, when using the excavator, the ore's mass is added to the system.
In my test the mass of the ship is 6.7t (picture1) and the asteroid mass is 178.4t.
The total mass of the system (ship + asteroid) is 185.1t (picture2).
If I remove 1.5t of ore from the asteroid and put it on the ship, the mass of the asteroid decreases by 1.5t and the mass of the ship increases by 1.5t, so the total mass must remain 185.1t.
Picture3 shows that the amount of minerals is added to the system, indicating an increase in the total mass of the system by 1.5t. I believe there is a bug here, because the game is keeping the mass of the asteroid fixed and adding ore mass to the system.
If disengage the ship from the asteroid, the mass will return to 6.7t + 1.5t = 8.2t (picture4)
Now, if I discard the ore from the ship and connect to the asteroid again, we can see that the game keeps the mass of the asteroid fixed because it considers the total mass equal to picture2, even with the asteroid without 1.5t of ore. (picture5)
The video showing the problem https://youtu.be/SRzGvAtCRrA
Additional info: apparently the game is resetting all asteroids to their original mass values when you fly them. Even on asteroids I mined out years ago. The PotatoRoid "mass" property in the save file is no longer consistent with the "currentMass" property of the ModuleAsteroidInfo module.