@dos I remember having a similar issue with my xubuntu many years ago. It'd happen randomly. Changing the system font to a different one always helped. Once a font got corrupted it'd remain in that state until reboot, but switching to a different font helped each time (yes, this could happen to the next font too)
@naugtur I expected something similar, but neither rebooting nor purging ~/.cache helped. I can still instantly reproduce it by switching font hinting level to None or Slight 🤨
@dos I remember having a similar issue with my xubuntu many years ago. It'd happen randomly. Changing the system font to a different one always helped. Once a font got corrupted it'd remain in that state until reboot, but switching to a different font helped each time (yes, this could happen to the next font too)