Not sure it it helps you, but might shed light on what happened. Aluminum can creep if held in a stressed condition over a period of time. In the aircraft industry, that is how they form stringers. They elastically bend them with tooling and leave them in an oven
at surprisingly low temperature for 8 hours. They come out plastically deformed. It is called creep forming. At ambient temp it just happens in months rather than hours (probably dependent on alloy and heat treat). Is suggests that forced the other way, it would creep back over time.