I have recreated your set up, however, no issue regarding DHCP leases shows up. The client device receives IP from the pool of addresses configured in the router, to which the client is connected to.
Could you answer the following questions:
Are both devices using different subnets?
Does the same issue reoccur with different RUT devices?
Did this happen with previous firmware versions?
Does this happen when load balancing or failover is disabled?