If you link a product (copy to another category) then delete one of the products, the price break and group table entries are also removed.
Around line 361 find;
In that block the following lines should be removed/commented out;
// LINE ADDED: MOD - Separate Price per Customer
tep_db_query("delete from " . TABLE_PRODUCTS_GROUPS . " where products_id = '" . tep_db_input($product_id) . "' ");
// BOF QPBPP for SPPC
tep_db_query("delete from " . TABLE_PRODUCTS_PRICE_BREAK . " where products_id = '" . (int)$product_id . "'");
// EOF QPBPP for SPPC
Price brakes and groups are not duplicated with each linked product so only need to be deleted when the final product is removed, which they are when tep_remove_product() is called.
Create a product with price brakes.
Copy that product to another category by choosing the link option.
Delete one of the products.
All price break data will have been lost.