Passing Local to Method allows it to be overwritten

The following script demonstrates this:

DefinitionBlock ("", "DSDT", 1, "RSACPI", "TESTTABL", 0xF0F0F0F0)
{
    Name(FCNT, 0)

    Method (CHEK, 2) {
        If (Arg0 != Arg1) {
            FCNT++
        }
    }

    Method (INR5, 1) {
        Arg0 = 5
    }

    Method (T5) {
        Local1 = 1
        INR5(Local1)
        CHEK (Local1, 1)
    }

    Method (MAIN, 0, NotSerialized) {
        T5()

        Return (FCNT)
    }
}

Local1 of T5 should remain as 1, instead it is overwritten in INR5.

I think this can be fixed by special-casing do_store to avoid writing back to the inner ref if target is an Arg that's anything over than a "true" reference, but I feel like I've missed something obvious?

Start by reproducing the issue with the AML script in the report and trace the argument/local handling through do_store. Determine why passing Local1 to INR5 permits the callee's Arg0 assignment to overwrite the caller's local. Done means the script returns FCNT as 0, showing Local1 remains 1 after INR5(Local1).