2S19 Msta-S , from what I could gather. The western ones (M109, Krab, PzH 2000, etc.) all have a turret with overhang.

It looks like you are using this one: ArctiClean 1

Which seems to be specifically made for this application and supposedly consists of “citrus and soy based solvents”. Aluminium is solved by highly acidic (like sulphuric acid) and highly basic (like sodium hydroxide) solutions. Which it really doesn’t sound like until it somehow broke down in a way it got more potent.

Was it maybe a specific thermal paste with a metal like mercury or gallium inside? Supposedly they can quickly dissolve aluminium. Of course, I don’t know what happened but maybe it was dissolved by the cleaning solution and then rapidly reacted with the Aluminium of the heat sink. If so, that would still make me wonder about the bubbling and foaming when the cleaning solution comes in contact with the left overs.