It seems that CDTs created in Appian and/or created from database tables then modified in Appian cannot be used with Generate New Action wizard in Record Types.
It also appears that we cannot use CDTs made from scratch in Appian with Generate New Action wizard in Record Types.
What is the root cause here? It seems as soon as you touch CDT from within Appian editor you cannot use it for Generate New Action wizards anymore (the Replace with existing object feature is not available or not working).
Please advise.
Related: Generate new action assistant Appian Community
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I also experienced this behavior. But I do not like the way of how these actions are generated, I do not care.
But still, that should work.