Some years back I switched to a modified paleo diet, but I struggled to shed pounds. Then I read something that prompted me to switch from Diet Coke/Diet Dr. Pepper to soft drinks sweetened with Splenda (Diet Rite or Diet RC). Within days I began shedding pounds. Since then I have noticed this same pattern several times. (I inevitably end up drinking aspartame drinks again because they are ubiquitous). I can’t prove it scientifically, but I suspect there is something about aspartame that hinders weight loss, at least for me.
You may be on to something. Many artificial sweeteners were never tested for insulin responses for non-Type 1 Diabetes. They work for patients who do not produce insulin, but have been found to increase insulin production in others. I suspect those with insulin resistence will continue to gain weight with certain diet sodas.
When I switched over to low carb, I cut out diet sodas. I observed that they stopped the ketosis process and slowed down weight loss.