This is not a language issue, but programmers who have made a mistake. The same thing would happen if an assembly programmer were not to set the correct registers or push all the arguments to the stack before calling an other function. The same thing would happen in most languages if a programmer declared their FFI incorrectly.

So Programmer/s have written function prototypes for the Fortran functions, but they have omitted the length parameter in that prototype. That is the reason behind this. It should have never worked to begin with but the way it was handled on the FORTRAN side things meant for single length strings it did not crash. GCC probably could have just made the change they wanted, but they don't want to break existing code right away that is technically already broken.

It boils down to some programmer/s doing the following.

int foo(char str);

instead of

int foo(char str, int len); // The correct prototype.

The prototype is you saying there is a function that exists with this name, return type and parameters. The compiler will then construct calls with what you have defined as the function prototype. Then when linking the linker just looks at the name of the called function and replaces it with the address/memory position of the function.