[synthesis5x]

Addressing the question of what uRPF has to do with this: it’s possible—unlikely, but possible—that Meta hasn’t been able to find the issue because while it looks like a faulty interface within a bundle (and it probably is), it could also be that an internal route has uRPF accidentally enabled and is receiving asymmetric traffic, causing it to drop packets on that path. It’s a possibility, but only Meta would know for sure. I included it in the title to give them a lead; it can really only be one of two things: uRPF on an interface participating in an ECMP, or an interface dropping packets at the hardware level within a bundle