That’s confusing - there is no default set column values with a form.
A form adds a row when it is submitted - that’s the default and only thing it does.
If you configure an onSubmit action, I don’t think there is any guarantee that it waits until the new row has been added. In fact, anecdotal evidence suggest that it doesn’t. This is why it’s generally recommended to not use an onSubmit Set Column Values to change values in a row that has just been added - it won’t always work.
Best practice is to pass all values with the form, using column values, special values or user profile values where necessary.