Why does a partial application have value restriction?

Question

I can understand that allowing mutable is the reason for value restriction and weakly polymorphism. Basically a mutable ref inside a function may change the type in

Answer 1

A 'let' expression is not a (syntactic) value. While there is a precise definition of 'value', roughly the only values are identifiers, functions, constants, and constructors applied to values.

This paper and those it references explains the problem in detail.