omit something but according to your explaination, you did not mention anyhing about ISP Y's routing policy.
Y's routing policy should effect both blocks in the same way however, and not be an issue - so long as they haven't decided to specifically route the block strangely. I'm actually dumbing down this problem as we obviously have more than just 2 peers and certainly more than 1 AS-hop at play here, but if I can get an answer to why this is happening I can reason it out from there.
In theory, someone else other than you can change BGP route update infomation, but I don't think it is the case or perhaps?
Theoretically people are not tweaking the routes, atleast thats what I am told. Oh well, back to pounding my head against a wall and watching the History channel. -- Jason Weisberger Chief Technology Officer SoftAware, Inc. - 310/305-0275 ...but the wicked shall do wickedly... --Daniel 12:10