Yes, doped cones with a ‘sticky’ feel are fairly common. I have sometimes wondered about long term change arising from dust sticking, though I guess it is negligibe.
I saw once that a video of dynaudio on how to clean tweeter. Their advice was to: just dont touch the tweeter. I totaly agreed whit them, first is not necessary performance wise, second it can be sad and expensive to make a mistake by trying to clean them.
I feel.the same for the mid-woofer of the 19 and the mid dome of all ATC product.
Interesting though that the newer 11, 19 versions with the metal grill are far less sticky than the older square versions, it looks like atc have changed the compound or applied it to the rear of the cone as they are also a more matt finish and the weave of the polyester is more visible, my old 19’s were glossy and tacky but my new ones are not any where as near…