dax syntax - calculate(table) vs filter context transition and return confusion

Yes, you've hit the nail on the head: "query tables" created with DEFINE TABLE cannot reference each other!

But variables can 🙂

I'm not sure if your query has evolved since your last post, but I would possibly suggest something like:

EVALUATE
VAR pml_id =
    -- pml.id = 3921
    CALCULATE (
        MAX ( 'Project Master List'[Id] ),
        'Project Master List'[DIR] = "CO",
        'Project Master List'[Initiative Name] = "Fleet Business Transformation"
    )
VAR mbm =
    CALCULATETABLE (
        'Measure Benefit Master',
        'Measure Benefit Master'[DIR] = "CO",
        'Measure Benefit Master'[PM IDId] = pml_id
    )
RETURN
    mbm