[popcornmix]

Found the issue!

It's because of my overscan settings.

Code:

#  1080p
  overscan_left=39
  overscan_right=39
  overscan_top=23
  overscan_bottom=24

Shouldn't I be able to use this anymore? I thought this method was better to use instead of the videocalibration tool inside kodi.

Actually I think you'll find "overscan_scale=1" is the critical setting.
Without overscan_scale, the other overscan settings will have no effect to kodi.

But, no. You should not be using any overscan settings in config.txt with kodi.

For best quality fix the problem on the TV. Check the display menu options and disable overscan (it may be called "just scan", "screen fit", "HD size", "full pixel", "unscaled", "dot by dot", "native" or "1:1").
If you can't do that then use the kodi calibration settings. That will result in a resize by the TV which will make the image less sharp.
Using overscan_scale will result in a resize by the Pi, followed by a resize by the TV resulting in an even less sharp image.

I'll try to discover why the overscan_scale stopped working. Are you sure it was #0430 that was the first broken build?
There were no related changes there. #0501 did have some firmware related display changes so seems more likely.