Did you ever figure this issue out, I have the exact same issue. Same scenario made my own cable and same results to the T.
I found a workaround.
You can assign SWR function to your CS key, pressing it will trigger an SWR test, which sends a low wattage carrier signal to the tuner and triggers an auto tune. This basically does the exact same thing, with one exception, on the FT-450 with the cable installed disables the onboard tune button of the tuner. Using the CS key leaves the manual tune function enabled on the external tuner.
Dave