I am no debugger expert, but what happens here is that the parameter is passed through a register (namely A), and not on the stack. Check the value of the A register (the register view is not included in you screenshots). I would expect it to be 0. Note that for functions with a fixed number of arguments and when the size of the last argument is one byte, the compiler will always pass this last argument through a A register (this behavior is documented in the compiler manual in this section: "Calling Protocol and Conventions" ).
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