Have you changed your manufacturing process by any chance? It is possible for flux or cleaning solution to make its way inside of relays during reflow and cleaning. Even if the relays are sealed, there are still specifications to watch out for, especially in lead-free processes.
If disassembling the relays and/or having a professional lab evaluate the contacts for contamination, watch for traces of organic elements.
Go to the product webpage on TE's website and take a look at the "Transportation, Storage, Handling, Assembly and Testing" datasheet (sections J through O) to see if you are within their specs yet.
Anecdotally: I spent months chasing down a very similar issue with what might have been the exact same model of relay. TE disassembled a few they had reserved from the same lot code at their Czech plant and found no issue. A third-party lab cracked open some relays from before and after the manufacturing process and found organic contamination on the contacts of the parts we pulled off manufactured boards. Finally, our contract manufacturer admitted to using a solder process that was hotter than they were supposed to use, then were spray washing them with liquid that was definitely too hot and high pressure. We believe when the relay cooled, it may have sucked residue in through the micro-cracks in the relay seal from the too-hot reflow.