The default value of the checkbox should work fine for new records, otherwise you need to review the custom field set up again.
For existing records, the checkbox value will most likely be NULL, I don’t believe the system will retrospectively update all existing records for you. My suggestion would be to utilise data migration or a suitable PLSQL script to correct the existing records.
Not sure if Find & Replace would work as well, you have to essentially check and then uncheck the checkbox.
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