I'm not a networker, but sounds like from a correctness standpoint, a problem on Fastly's end -- they're reusing frontend IPs for distinct sets of machines, and traffic directed to the 'wrong' PoP is dropped hard rather than attempting any kind of internal routing

Of course that kind of routing would create a potential bottleneck for an attacker to exploit ("simply" force traffic to the wrong IPs to the wrong PoP, assuming $attacker had this level of access to the backbone), but that's the problem Fastly are supposedly paid to deal with

Their scheme is fine and dandy with a protocol like DNS where UDP retries are transparent and TCP is a tiny fraction of weird traffic, but for business applications handling credit cards, surely the occasional RST is already too many

Or another way to look at it, basically they're saying their IP addresses are special snowflakes and actually the full address includes the route, and source networks are wrong for assuming things work the way they're supposed to everywhere else on the Internet