Yes exactly what i'm saying. Based on this article,
Wearing a belt allows you to lift more weight, and even with the same training weights it increases muscle activation in the quads and hamstrings without decreasing abdominal activation.
So it seems to me my quads are getting overwhelmed whem using a belt. Too much force to absorb. Or the hamstrings are overpowering quads. Either could be happening i guess
The reason why you have worse form with a belt is because you use heavier weights because you feel stable enough to use them due to the increased abdominal tension (because of the belt). The CNS translates that into "this is safe(r) enough for this heavier weight" so then the leg muscles are being overloaded (since they aren't used to that heavy weight) and you start to use your back more etc to compensate.
If you want to see what the belt does or doesn't, compare a squat that you're able to do without a belt with the same squat done with a belt.
So say 140 kg beltless with 140 kg with a belt.