Truetype font has a feature which allows you to embed bitmap font for specific font size, to optimize the rendering results on screens. But using such fonts in Pillow will generate some strange result.

Example:

In this case I'm using a modified version of SimSun, which contains embedded bitmap fonts, for all size between 8px and 24px. In the image, first line is using 24px font and the second line is 28px.

TestCase is available (Contains SimSun font, testcode and the result above).
This test can be reproduced on both windows and linux.