It's not truncation...
Documentation of the ACO0103E command says:
"This message displays the command that was entered, up to and
including the option that was detected as invalid for the command."
So, the /Stopat is being called out as invalid. Options may be valid
unto themselves, but may be invalid in certain contexts and
combinations. I don't work with that TDP, but you or someone more
familiar with it may be able to deduce what the issue is in that regard.

    Richard Sims