I notice that after deleting the last record and then adding a new record, the Mol_ID does not use the deleted record's Mol_ID. Why?
Mol_ID's are only guaranteed to be unique, not sequential mainly because the Mol_ID in an MSI file can not be deleted by record deletion. If you want to delete unused Mol_ID's and compress, you can export your ChemFinder database as SD file and import the database back to ChemFinder. This should eliminate Mol_ID's of empty records.