I increased the logging verbosity and requested a build for this host again.
accept: 192.168.1.159:42492
Rack::Handler::WEBrick is invoked.
c5c4d669 [I] Started GET /v2/features
c5c4d669 [D] verifying remote client 192.168.1.159 against trusted_hosts ["foreman.our.net", "zsinf08.our.net"]
c5c4d669 [I] Finished GET /v2/features with 200 (37.8 ms)
[D] close: 192.168.1.159:42492
That was all that I got in /var/log/foreman-proxy/proxy.log