This has been debunked. It’s not that diet soda “causes” weight gain, the studies show that people eat more because they think drinking diet soda gives them a license to do so.

The old I am going to order a double quarter pounder with cheese, large fries, and a Diet Coke.

https://www.diabetesdaily.com/blog/2014/06/do-diet-sodas-cau...

https://www.inc.com/minda-zetlin/diet-soda-weight-gain-metab...