When I was cleaning a heatsink from an older laptop, the aluminum, and I think also a bit of copper, started to disintegrate very quickly and sometimes violently. I used a cleaning solution that was made for removing thermal paste and cleaning CPUs and PCBs. After some time, the only thing left of the heatsink was some grey powder or ash. I have cleaned many heatsinks and CPUs with it before, and this never happened, and I can’t reproduce it.
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I can’t get your images to load, but to me, a materials chemist, it definitely sounds like you’ve managed to dissolve the aluminium and copper by using an acidic solution. Presumably, the cleaning solution contains some electrolyte that should help dissolve oxides, but if the solution is corrosive enough to oxidise the aluminium and/or copper the electrolyte will make the reaction more aggressive by rapidly dissolving the protective oxide layer as it is formed, such that the aluminium/copper is further corroded.
To be fair, this is just speculation based on what I’ve read here. I could maybe give a better analysis if you let me know what solution you’ve used, and what the heat sink/paste were made of, and if I can get the pictures of the resulting product (“ash”) to load.