Load balancer using wrong origin name (SNI?)

We have (for the purposes of this question) two hosts with DNS records thus:

A      a-1.example.com
CNAME  ac.example.com
A      b-1.example.com

The origins are serving https on port 443 with nginx. Origin a-1 serves differently depending on hostname (a-1, ac, localhost).

We’ve configured a load balancer with pool p and origins a-1 and b-1.

When requests hit b-1, all is well. When requests hit a-1, however, SNI is apparently requesting that ac respond.

Any suggestions how to approach this?