How can we use "uniqueid()" for an existing spreadsheet created by Google Forms?

[hichimi]

I am running a local athlete club team and using Google Forms to accept new membership registrations.
I recently created an AppSheet App using the spreadsheet where the registration form results are stored.

Now, I set up a column both in the spreadsheet and in the AppSheet that sets a unique ID for each record. I set the "ID" column at the last in the spreadsheet so that it won't conflict to the Forms, and put random IDs to the existing records. I did "regenerate" in the AppSheet so that it captures the new column. Once I confirmed the "ID" shown in the AppSheet "Data" configuration UI, I put "uniqueid()" in the "initial value" field.

What happened was, a random ID was created when I add a new record (member) from the App, and the ID was also populated in the spreadsheet at the same time, but that was only the records from the App, not for the records from the Forms. When I inserted a new record from the Google Forms, the record didn't get the ID and the cell was left empty.

There are some requirements that AppSheet form can't fulfill, so we need to use the Google Forms. How can we solve this issue?

Have a great new year, everyone! Love from Japan.