In the Expanse universe, both books and series, ships came to the space where their velocity is forcefully limited. This is allegedly because of alien station justly considered fast-moving objects as dangerous. But Epstein drives are so good exactly because of extremely high speed of cast reactive mass. That is, their exhaust is extremely dangerous too.
Why wasn't speed of particles in engine exhaust limited in the "Slow zone"? Or it was, but I missed it? I think not, since limited exhaust couldn't push huge mass of ship.
UPDATE: Thanks to Null for exact pointing to original text. Now it is obvious that it's a plot leak. Epstein drive had exhaust speed near 11 000 000 m/s, and the whole idea is simple, to accelerate particles with set of magnetic fields. If you cast a kilo of reactive mass with 11 000 kilometers per second, your 1000-ton ship get 11 meters per second. And if you could accelerate particles only for 600 m/s, ummmm... I have bad news for your 1000 ton ship.