Canson Platine edges discoloring

I have a Platine print was hot mounted on rag board with acid free tissue 2 years ago, but the edges are now discoloring. I have one other print doing the same thing. Only these 2 prints were hot mounted, I usually use corners. Tried to send a message to Canson with no response and cannot find a contact phone number for them. Also checked with the maker of the tissue and they are stumped as well. Any ideas are welcome as well as a contact phone for Canson support. Thank you.

I don’t think this is an issue with Canson and more an issue with the mounting material itself. Double verify that the adhesive was archival certified.