Problem

Your ssh config Includes another config file with hosts. When trying to connect to a host in that referenced config the host is unknown.

Example

Host example.tld
    Hostname example.tld
    User myuser
    
Include ~/some-other-ssh-config

Solution

Put include statements first as those will be interpreted as sub-configs to the Host entry.

Modified example

Include ~/some-other-ssh-config

Host example.tld
    Hostname example.tld
    User myuser