Not to forget the Fortran language standard does not get into implementation details and considerations such as variables on the stack vs heap are not relevant to the standard.
The key aspect is undefined.and per the standard, it makes it nonconforming to reference an object that is undefined.
The instructions with PRINT on lines 5, 6, and 7 in the original post thus do not conform and the program behavior can be viewed as processor-dependent or better yet, not to be “read much into”.