Thanks for the suggestions. The cap measures perfectly fine, but so do a whole box of few hundred old caps that I have, most of which rattle and measure okay. And for those I definitely know they shouldn't rattle when new. So I wouldn't rely on measurement alone, I'm more curious if that's at all possible to be normal for the type or indicates a problem, such as bad seal maybe. If anyone has the same or same type, would be great to share. Shame I only ordered one, so no base to compare.
Oh, and I think it's safe to say it's not fake, it comes from DigiKey. Also the date code is 2051 which if I'm not wrong would make it just under an year old, so not a problem there either.