This happens in the version 3.0.9.GA-SNAPSHOT containing all commits up to

f202aaa Configure TCP channel even when connect returns immediately

When making short connections on 100-120 threads in parallel, between 1 and 5 of 1000 channels are connected, but when checking isOpen before first write, they return false. No reading has occured on those channels either at that point in time.

These might be channels which are in a similar state to those which did not return a peer address previous to the fix of XNIO-213.
The connecting code is the same as given in XNIO-213.

Linux (CentOS) seems not be affected by this behaviour.