A quick search does bring up this page, which mentions an observational study where people who drank more diet soda gained more weight. https://www.webmd.com/diet/features/diet-sodas-and-weight-ga...

But, again, "diet soda causes weight gain" doesn't explain how drinking more diet soda would lead to weight gain. Whereas "people who drank more diet soda also ate more (than those who drank less diet soda)" seems both plausible, and would explain the result.