I found out that I can reproduce this behavior by setting QT_DISTANCEFIELD_DEFAULT_BASEFONTSIZE, QT_DISTANCEFIELD_DEFAULT_SCALE and QT_DISTANCEFIELD_DEFAULT_RADIUS variables. With these, the problem occurs even on Windows.
Not sure whether it's the same problem or not, but if QT_DISTANCEFIELD_DEFAULT_RADIUS is NOT DIVISIBLE by QT_DISTANCEFIELD_DEFAULT_SCALE, the problem occurs 100% of time (some letters will disappear).
Example:
export QT_DISTANCEFIELD_DEFAULT_BASEFONTSIZE=128
export QT_DISTANCEFIELD_DEFAULT_SCALE=12
export QT_DISTANCEFIELD_DEFAULT_RADIUS=96 // If I set this to 95 or 97 or anything not divisible by 12, it will break.
Can someone explain me what these distancefield environment variables do and whether this is an intended behavior?