When glyph fallback kicks in, the previous character is always taken from the fallback font needlessly. This seems to have been introduced in 3b4d361c21a1289b664cbcb9c15751d99ef6348d, but nGraphemeStartPos does not seem right to me, as it is the start of the previous grapheme not the current one.
that's definitely not what I want to happen
Caolan McNamara committed a patch related to this issue.
It has been pushed to "master":
Resolves: fdo#66042 get the bounds of the current grapheme
The patch should be included in the daily builds available at
http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.