OK, this might be useful:
Take the string pattern "WHwh".
Starting with a font size 400, progress down in font size incrementally.
At some font sizes, there is no truncation.
At some font sizes, there is truncation of the "W" only.
At some font sizes, there is truncation of the "w" only.
At some font sizes, there is truncation of the "W" and the "w".
Doing this, it looks to me as though truncation might be occurring when the following "H" or "h" boundary moves over the preceding "W" or "w", respectively.
It's like the proportional spacing might be a bit out of whack at certain font sizes.