Welcome to TPT.
Interesting and I do not know the answer but I do have a personal experience to share. I set up a new tank about 1.5 years ago with 2/3 of used AS and 1 sealed bag of 4-5 year old AS. That tank was ok for about 3 months and then it started one type of algae bloom after another with no end in sight. When I started pulling all of the substrate out, I noticed layers of distinct discoloration - same of it was almost white in color.
I do not know if AS "went bad" or if that bag was contaminated or what. What I do know is that I have 7 other tanks in the house.
My problems with that tank could have been totally unrelated to AS, I've done stupid things in the past.
Hopefully, other members will pitch in.