Here’s how you determine the right course of action. First, adjust the size of the subdial. Its area should be equal to 10% of the main dial area, less if the effective area of the dial is reduced by frivolous chapter rings. Next, superimpose the 6 where it would normally exist, if the subdial wasn’t cutting it off. Determine how much of the number’s area would fall inside the subdial (can use a hand-drawn mesh or CAD for this). Anything more than 66% falling inside the subdial and a dot is preferred to a cut numeral.
Of course I‘m kidding around.
If the movement is properly sized for the dial, it doesn’t really matter, because the sub dial will be well positioned and proportioned. The cut of the 6 is secondary to the overall visual balance.