New to most of this but couldn't you just salt the hashes being used per-router?

Yeah I think you could seed each router's hashing with something unique like a mac address hardware ID but you could also just use different hashing at each tier in your network like using source IP + dest IP on one tier and source IP + dest IP + IP protocol on another tier. This should achieve the same result.

Yeah but then you need to keep coming up with new ways of hashing things as you add tiers. It's easier to generate a new salt than it is to figure out a new but still-valid way of hashing the packets.

Practically speaking this isn't an issue though. You don't see many spine and leaf networks that are deeper than 3 tiers(edge, distribution, ToR) which means one link upstream and one down. It's not like people build networks arbitrarily deep.