Если оба интерфейса настроены Active/Active, а не Active/Standby, то исходящий трафик распараллеливается между всеми интерфейсами IPMP группы. Настрой второй интерфейс как StandBy и наверное настанет счастье.

ищи документ "IPMP Rearchitecture: High-Level Design Specification"

в Active-Active IPMP может отвечать 1) с любого активного интерфейса 2) использовать в исходящем пакете любой из IP адресов в качестве Source address

3.2 Outbound Load-Spreading
Currently, the IP module load-spreads outbound traffic for each IP destination address across any
active interface in the group, where active is defined as an interface that is not marked FAILED,
INACTIVE, or OFFLINE, and has at least one UP address14. This will be unchanged in the new
model.
3.3 Source Address Selection
Currently, if the application does not bind to a specific source address, the IP module will select
a source address from the set of addresses in the IPMP group. The purpose of varying the source
address is to affect inbound load-spreading. Specifically, in the current model, all addresses hosted
on a given interface are associated with that interface’s hardware address. Thus, currently, if one
connection uses a source address on ce0, and another a source address on ce1, then each connection
will use a different interface for inbound traffic. It’s important to note that choice of outbound
interface is independent from choice of source address. Thus, it is quite common to have
a connection which sends packets through ce0, using a source address hosted on ce1, and thus
receives the response on ce1 (this is necessary to spread outbound load when the set of available
source addresses is smaller than the set of available outbound interfaces).