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. for this problem:
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.
DaveIf there are better solutions, I'm curious to hear about them,