[cowmoo728]

It is possible (likely?) that the observed gut flora changes interfere with normal metabolic function, causing long term weight gain.

2020 - "future studies should consider the metabolic pathways of different artificial sweeteners. Further (long-term) human research investigating the underlying physiological pathways of different artificial sweeteners on microbiota alterations and its related metabolic pathway is warranted to evaluate the potential impact of their use on body weight control and glucose homeostasis."

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7817779/

[andrewmutz]

Yes, there has been some fascinating research coming out suggesting that the gut flora composition can have a causal effect on obesity. For example, if you transplant feces from overweight humans and normal weight humans to mice, the mice will gain (or not gain) weight, depending on which person the feces came from:

https://www.science.org/doi/10.1126/science.1241214