If a host does not have hostname and only exists in .ssh/config as a following entry:

Host LINUX_BEHIND_JUMPBOX.CC
    HostName 10.11.11.11
    User gw
    Port 9022
    ProxyJump BASTION.CC

ssh connect with:

ssh LINUX_BEHIND_JUMPBOX.CC

would work.

However:

xxh LINUX_BEHIND_JUMPBOX.CC

does not work.
This is caused by fact that underlying ssh command warpper tries to connect to lowercase version of this hostname, but OpenSSH treats those entries as case sensitive. Converting hostname to lower case happens in: xxh.py in lines which retrieves hostname: url.hostname which returns lowercase result.
Simple switch to url.netloc helps with my case, but I am not sure if this doesn't break anything else.