2017 3 Oz. Reverse Gold-Plated Pure Silver Coin - Whispering Maple Leaves

Based on your images, it looks like corrosion rather than tarnishing or toning.

After three years, it's unlikely the RCM will do much but you have nothing to lose by calling them and asking.

What type of environment has been used to store the coin?

I got this one from the dealer and it was a Pre-order, meaning they were getting it directly from the Mint.
I keep it in original packaging, rarely looking at them. It's in the drawer in my office. Just a regular home environment. This one I guess wasn't disturbed for more than 2 years. So it was a bad "surprise" for me discovering it.
Another thing, I found the evidence of handling this coin with bare hands - two symmetrical light toning on the edge.

The last sentence totally may explain it.
When Mint accept the return there is no way to know if someone opened the capsule and handled the coin with fingers. It takes time for coin to tarnish at the point of fingers contact. So when I got the coin it still wasn't showing.

Got an answer from the dealer I got the coin from with the interesting statement:
"A large percentage of gold plated coins that are over 5 years old are starting to tarnish like your coin."
Anyone has an opinion?

Another question, does anyone know the email of the quality department of RCM?
They have only web form to contact them, and I can't attach any pics to it...