Ok, I think I know what this is... I had to change the behaviour of the <d> tag to insert a zero-width space afterwards since using <d><e=myEvent><d> just compounded on the last character. So the character spacing being set to -0.1 is basically causing double character spacing by moving those zero-width spaces.
Looks like I was accounting for that in a few places, but forgot some... should be fixed, will publish this asap!
EDIT: Update should be out, look out for v1.14.11!