Propagation without central body

Hi all,

I am trying to propagate orbits around Lagrange points and in order to validate the propagation, I use the JPL JWST Horizons’ ephemeris as reference and starting point of the propagation.
I face a very strange behaviour. Indeed, propagating without central body and with only Earth, Moon and Sun single body attractions, in Solar System Barycenter reference frame I end up with a position error of more than 2km after 1.5 days (I also added inertial acceleration but for SSB, I checked, the acceleration is indeed 0.). Introducing Jupiter perturbation reduces the error to ~200m.
The very strange thing is that I compared these 2 propagations with a classical propagation with an Earth-centered and luni-solar third body propagator (without jupiter then) and this propagator gives the best results.
The propagation without central body and Jupiter added is the second best at ~25m of the classical Earth-centered propagator (cf. plot).
I do not succeed to explain this behaviour. In my opinion, single body propagation (with Earth, Sun and Moon single body attraction) and earth-centered J2000 lunisolar propagation have to be equivalent.
The inertial acceleration of earth-centered J2000 reference frame seems to me (correct me if I am mistaking) taken into account in the differential acceleration of the solar 3rd body perturbation.
If you have any explanation for this, I would be very glad to hear it.
Many thanks in advance !

Please find attached the python script (I use Orekit with Python through Jpype) used to generate the data, and the reference OEM (sorry, I was mistaking, the test case was on Orion DRO instead of JWST but I have had the same behaviour with JWST case). Hope this could help.

valid_propags_orekit.py (7.2 KB)
CCSDS_OEM_Orion.txt (382.7 KB)