It is also not permanent. The orbits of the debris would intersect the point where they depart, which is pretty close to the engine. Basically the engine would be hitting itself with everything it fired.

Therefore, the logical thing to do is to put it on the correct side of a mountain, to shield the engine. But that would also collect all the debris. So it would generally only be in orbit for one orbit.

That makes it more targeted than some of the other unfriendly things you can do in an airless planetary environment: https://news.ycombinator.com/item?id=35862424

(To be honest, I think on any planetary body without an atmosphere, long term everyone is going to have to dig in to the planet, and to a non-trivial degree, too, not least of which is the complete indefensibility of surface installations.)

The expansion of the gas after it leaves the nozzle in vacuum would give the particles an additional kick. I'm not sure if their orbit would still intersect the engine or effectively boost higher.

I left myself some wiggle room in the phrase "where they depart" for that reason. It won't all be a straight line out of the rocket motor or whatever is pushing, because in the first fractions of a second the gasses and the particles can interact and bash each other into slightly different orbits.

However, that will dissipate quickly and you'll certainly be looking at a set of orbits that all pass through something relatively close to the origin. They're not going to be interacting for the first time a quarter of the way through the orbit and bouncing around a lot there.