Pluto was discovered 85 years ago and has on a 248-year orbit. It has not even been known to humankind for half a trip around the Sun yet. Calling the orbit “tranquil” is highly speculative - its planetary system sure isn’t tranquil by any stretch of the imagination, it is highly chaotic. We don’t even know for sure whether it is capable of completing one single, full stable orbit; the observational data isn’t there. Heck, we didn’t even know how many moons it had until New Horizons got close to it.
We understand orbital mechanics VERY well. Maybe not perfectly, but enough that Pluto’s orbit is predictable, from what we’ve seen, too well more than 40 revolutions.