Comment 7 for bug 614693

Viewing the sequence 'äëïöüÿÄËÏÖÜ' at very small point sizes (where the height of the umlaut is under 2 pixels) shows that some of the character have heavier part of the umlaut as the bottom pixel of the umlaut, and some have the top pixel being heavier.

Based on that, I'm unsure that hinting is the cause, but it might be that in the unadorned glyph there is a misalignment in the placement of the diacritical sub-glyph.