Hi Dan,
Strange indeed. I never used bCNC, so I would recommend to only stick to M3/M5 G-code commands while investigating this, instead of that slider, just in case (to remove one parameter from the equation)
If you type M3S24000, do you get 5V at controller output / VI input ?
If $30/$31 are properly set to 24000 and 0, there is no good reason to not have 5V on PWM.
If you don’t see that, unplug the PWM on the VFD side and retest. If you still don’t get 5V…well we’ll see. If you do after unplugging, that would mean a weird electrical interaction with theVFD.
Only once you have made sure that PWM output is fine and under control, proceed to investigate why the VFD would not react as expected. A quick and easy step would be to reset the VFD to factory settings (via PD013) and reapply the PDxxx values you mentioned. Sure, it should not change anything, but it will rule out the possibility that while investigating at some point you changed something ELSE and do not remember doing it 
if the spindle does not start when sending a non-zero voltage to ACM, you might want to check this little switchjumper on the VFD (last pic), , assuming you have a Huanyang VFD or similar