After the fix for JDK-8091832 we will render using non-integer scales on Windows platforms if the UI scale is set to a non-integer value, for example, 125% or 150%. With a non-integer render scale, the font spacing is incorrect in some cases. It looks like an accumulation error possibly caused by rounding or truncating at the wrong point.
It is most noticeable with smaller fonts and when using gray-scale AA. The attached test program can be used to demonstrate this. When run at 100% or 200% scaling it looks fine. When run with 125% or 150% scaling, the problem shows up. It is most noticeable at 125%.
If run on a system with a desktop UI scale of 1, you can use the following command to reproduce it:
$ java -Dglass.win.uiScale=1.25 TextSpacing