That sounds unusual for Bilsteins. I hope the replacement is better. After all, it is just one that was no good and that is most likely a one-off manufacturing fault.
But reputation isn't everything. OME dampers seem to last about 3 months. Funny, since they are reputedly Monroe in disguise. I have Monroes (Belgian ones) that I fitted in 2000 and they don't leak.
But I am thinking of replacing them with Ironman (also Australian) shocks which seem to last for ever.