The matching between the CMS user and the CiviCRM contacts relies, like most "relationships" between entities in CiviCRM, on an additional table that maps the IDs between the two entity types.
For the Contact to CMS User match you're looking for the civicrm_uf_match table. This table has the following columns:
ID - this is the ...
Are you using phpmyadmin to export the old one? You can check the box for ADD DROP TABLE which should solve this. There should also be an option to disable key checks but the drop table might add that automatically.
Using mysqldump on the command line it does this by default, and also by default it disables key checks.
Are those real contacts? Can you check the created date against the contact? It shows up at the bottom when you view the contact or you can grab from civicrm_contact.created_date using mysql.
If the dates are changing than it means those contact are created via Bots. Could check all the forms which are accessible to anonymous user have recaptcha installed ...
Welcome to SE CiviCRM
You will need the following MySQL server configuration:
add above configuration in your my.cnf file.
See mysql requirement for CiviCRM
You can safely ignore this setting unless you're hitting scripts in civicrm/extern or civicrm/bin or using cv directly. In that case I'd recommend switching to routes that go via WordPress e.g. the REST API or wp cv instead - they are much more reliable in WordPress.