Craft CoM suddenly displaced

@AndrewGarrison thanks for the detailed answer, a couple of players were having a lot of issues with this and I'm think they're going to be happy to know this.
Again, thanks for the explanation in detail :)

Craft stability increases with mass and inertia tensors (angular mass), so if you use massScale of less than 1 then you will have parts that are unrealistically light, yet still adding the same force they would at their normal mass. The wings are the most sensitive to this issue and they are actually a bit too light to start with. It doesn't have to be the wing that is mass scaled. Any part that is mass scaled will increase the likelihood of unstable behavior across the entire craft. We're still thinking of what we can do to improve this, but in the meantime, use mass scale with caution.

I think the issue with the craft you have here is related to the huge negative mass that's causing a strange CoM calculation. As mass is reduced, the negative component of the CoM calculation starts to move the CoM upwards too much. This is due to the fuel tank with the negative fuel.

@AndrewGarrison I don't know the details about how the craft was done (it's not mine) but the aerodynamics errors show up in other crafts. If the 0 mass is the cause of the issue maybe a solution would be to make 0 mass in the tinker panel be a value close to 0 instead of 0

You have three fuel tanks with negative fuel resulting in negative mass, which certainly confuses the CoM calculation. There are also 86 parts that are mass scaled to 0.

@AndrewGarrison hm, there're some aesthetic parts with 0 mass in the airplane I'm making now but the weight is reallistic, it's not like 1kg weight or something similar, may that be the issue?

That sounds a lot like this issue.

@AndrewGarrison there seems to be a major issue with aerodynamics.
What happened to the witch is happening to a lot of planes, as they approach a certain speed (the speed is not always the same and in mobile the behaviour is totally different) at low atmosphere they start to glitch a lot with one or more of this things happening:
- Sudden acceleration to 1k+m/s
- Discrete rotations (like it changes between two fixed positions and the control surfaces loose control)
- All parts going to random offsets for a couple of seconds (up to 30m offsets in random directions)
- Impossible to control plane/craft

I looked into this issue today and I think what's going on is the inlets actually have a bug where they briefly report 0% occlusion while flying. This causes the engine power to spike, which causes a sudden spin. I have fixed the bug with the inlets, but this craft does still gradually work towards a high CoM and eventually does start to spin.