So, I just posted that I thought that fixed it. And it does. A little. It fixes D6s. It doesn't seem to fix D10s or 8s. Or 4s.
It looks it applies Kerning to font size X and above, minimum is size 8. For the record, I tried increasing the size and it didn't affect the success/failure. Sorry.
(This explains why I thought I had tried it before without success, but I just didn't try all the permutations. In good news, testing has confirmed that you can apply font settings retroactively on the codes which wasn't explicit in the guidance.)