This is not related to substrate but to the way refraction is evaluated.
However we do want to improve the that part and already have: (A) by default we have distortion follow the material properties (F0/IOR/refraction assuming thick mesh) while (B) being overridable on the root node. And (C) We also have rough refractions available with Substrate. The thin shell behavior is also something in the todo list but no ETA.