Color/pattern shift?

[WeinerWrangler]

My 4 y.o. tweenie black and tan, Oscar has "non-regulation" coloring on his head. When we got him as a 6 week old puppy his black mask looked normal, but within 5 months it "slid" back.. now his head and ears are tan, with a faint pattern left from the mask. The black on his legs retreated as well. He is otherwise a perfectly normal "pet quality" purebred.

Has anyone ever seen this before or know why it happened?