On release 4.12 ISO 

------------------
# coreos-installer iso kargs modify --append console=tty ../4.12/agent.x86_64.iso 
# echo $?
0
------------------

So it runs with success!

On release 4.14 or 4.13

------------------
# coreos-installer iso kargs modify --append console=tty ../4.13/agent.x86_64.iso
Error: No karg embed areas found; old or corrupted CoreOS ISO image.
# echo $?
1

------------------

Similarly,

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# coreos-installer iso kargs modify --append console=tty ../4.14/agent.x86_64.iso  
Error: No karg embed areas found; old or corrupted CoreOS ISO image. 
# echo $? 
1
------------------

I don't see the failure from payload,

ISO of 4.13 or 4.14

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# coreos-installer iso kargs modify --append console=tty /root/.cache/agent/image_cache/coreos-x86_64.iso

# echo $?
0
------------------

So there is failure.

I see that as, 4.12 was based on RHEL8 whereas 4.13+ are based on RHEL9 so there is 
something behind it as change to AgentBasedInstaller ISO generation.

4.13    

    Always trying to customize agent-based ISO

    1. Create ISO and try to customize.
    2. Ref : https://url.corp.redhat.com/ABI-ISO    
    3.
    

    Fails to customize and same time make USB out of it.

    Shouldn't be failing.

Ref : https://url.corp.redhat.com/ABI-ISO