...
To prevent the occurrence of this problem for the single record update operations on the already existent records with empty "order" field we need to run these updates at any time:
UPDATE {tenant}_mod_source_record_storage.records_lb SET "order"=0 WHERE "order" IS NULL;
UPDATE {tenant}_mod_source_record_manager.journal_records SET source_record_order=0 WHERE source_record_order IS NULL;
Where {tenant} - the owner (tenant) for which you want to run the update.