In some cases the kerning between characters differs for the same group of characters on the same line

In some cases the kerning between characters differs for the same group of characters on the same line. In the example there is a line like:

RW RW RW RW RW RW

Note that the first group of "RW" has different kerning to the others in the line (except maybe one odd instance later on).

This seems to be related to harfbuzz-ng as it works when switching to the old harfbuzz