Hey here !
this is a “common” issue already adressed here : https://devtalk.blender.org/t/curve-to-mesh-node-even-thickness-feedback-thread/27271
Thanks to Kuboa in this thread : Why is my profile bevel distorting like this?
you can check those links : Curve-To-Mesh with Even-Thickness Node-Group (Geometry Nodes, Blender 3.0)
https://blender.stackexchange.com/questions/260113/uneven-profile-curve-thickness-with-geometry-nodes-curve-to-mesh