Explaining the result of experiment---Aluminum foil heated in boiling KMnO4 solution
Potassium permanganate solution tends to decompose with time, especially those which are not acidified. Just let an aqueous solution sit in a glass bottle and it will leave a brown film of tenacious manganese dioxide on the surface at room temperature. If water has any organic material, it will become brown and tap water is not pure water. It decomposes on the skin as well leaving a brown stain. You Al surface is no different, with a thin deposit of manganese dioxide, it may appear yellowish-golden.

Long time ago potassium permanganate was added to water wells to disinfect water. Of course it would decompose with all the organics. If you are well after three years, what is the problem?