I don't know what to tell you really.
I would not be the authority on this ....
However, I noticed on my 07 450 fsr when I replaced the oil filter with an aftermarket one, (one that actually came with a replacement o-ring) the replacement o-ring was different from the stock one.
Basically the stock o-ring (on mine anyway) was not just a round circle, it had a part that went around another bolt I think.
Therefore on mine I did not replace the o-ring (plus it looked ok still)
Could it be something like that? Did you use an aftermarket one or a gas gas one?