I usually avoid using On Submit, especially for something like this. The newly added row may not fully exist yet when the Set Column action tries to run. I always set values like that using one of the Columns Value, Special Value, or User Profile components. There’s also a trick where you can add an entry component, set the default value, and then hide it with a visibility condition.
Make sure you don’t put those defaults in a container and hide the container though. I had issues trying it that way like 2 months ago, ended up having individual visibility for each component.