In real life when the rocket nozzle too much overexpanded the flow of gases literally separates from the nozzle wall. This happens due to low pressure on the nozzle exit, the atmosphere pressure separates the flow of gases from the nozzle wall until the section of the nozzle where exhaust pressure will be about 0.1-0.4 of ambient pressure.
And this causes deadly vibration, nozzle bending, and unstable flow, and because of what it is interesting to see this feature in the game. The engine will get damage (we already have damage system in the game, the parts get damage when they are overheated) when working in the overexpanded mode. This also will make engines not possible to run in really low thrust modes (10-40%) on sea level due to low pressure on nozzle exit.
A great example of nozzle bending is the start of RS-25, then the pressure on the nozzle exit too low. You can see how RS-25 much nozzle bending in this video - https://www.youtube.com/watch?v=hDCCBgppG4s