That usually means it was overheated somewhere in the supply chain. Aspartame starts to break down around 30C.[1] So it doesn't take much.

[1] https://www.mdpi.com/2673-9976/40/1/35

Interesting for me it's the opposite. Since they haven't fully disappeared I get one whenever it's available. Next too it a zero is far too sweet and a normal one is gross.