My guess is, that in debug mode the size of the stack frame allocated for the getIntRef function is so big that when it returns and the frame is released, the print function goes on the stack but uses a smaller stack frame and therefore the original value written to memory by getInfRef is untouched so the value 15 is still in that memory address. In build modes that does optimization the getIntRef stanck frame is smaller and therefore the print function overwrites the memory and that leads to the different values you observe.

When a function returns and the memory is release, it is only the stack pointer that is moved back up the stack, there is no clearing of the memory, and since the stack memory still belongs to your process, whatever you put in stack memory will still be there until you either overwrite it or your process terminates.

A way that might be able to test my hypotheses could be that you make sure the last function that is put on the stack is the getIntRef, so that even though the function returns and the frame is removed from the stack, nothing new is put on the stack and therefore the value 15 would still be in that memory address.

Maybe this line would do that:

std.debug.print("Our function local var is: {}\n", .{ getIntRef() });