The specification says:

If a pre-5.0 mongos or a load balancer backing a pre-5.0 mongos is selected, the effective read preference is {“mode”: “primary” }``.

The problem is that for a load balancer, drivers don’t do SDAM so they don’t have a wire version during server selection to compare against.

It would be reasonable to assume load balancers are only backing 5.0+ servers, and specify that behavior in the CRUD spec.