Would try latest 6.42.7 (make sure firmware is updated on System > Routerboard too). Failing that, would write support, linking to this post.
On hEX S there are only two PoE-out modes - off and forced-on. Auto mode is not configurable:Did you force poe to on on the hEX S when powering the hAP ac2 or left it on auto?
/interface ethernet poe> set ether5 poe-out=auto-on failure: This model does not support poe-out auto-on mode