That washer serves to lift the oil filter up but I don’t think it has anything to do with the seal. Did you have the washer #3 there, which may be critical to prevent leakage from around the bolt? Take a look at the side of the canister - is there an oil leak running down the side suggesting the leak was from the upper gasket or is the side clean and the leak came out the bottom?
If you end up needing that washer, I think I have one. I had heard the warning stories about that washer getting lost and bought one only to realize that it isn’t necessary in the top bolt filter canisters -it doesn’t fit down in there.