By chance, is this a T or corner joint?
You mention the base metal is 20 mm thick. If the configuration is either a T or corner joint it makes sense that they are specifying a PJP in the corner with a fillet weld. Maybe I said that wrong, not that using a square groove of 1.6 mm PJP in the corner with a reinforcing fillet, but just that perhaps the designer is trying to ensure proper fusion in the root of the fillet is achieved.
I agree that it is prudent to run a couple of tests to ensure you can achieved the required penetration with the square groove. I don't know what process you are using, but is you are using GMAW in the spray transfer mode, you should be able to produce a single pass fillet weld with the 1.6 mm penetration specified.
Just saying - Al