Product url_keys nulled after some time

Setup: Magento 2.2.x, OpenSouce (Magento Community Edition)

Products were imported from Magento 1,

• url_path attributes were cleared
• all url_key values are unique (both products & categories)

After 48.0h, some of product url_keys gets nulled (NULL value in catalog_product_entity_varchar), without ANY operations done by the Client.

Has anyone experienced such issue before?