Custom login screen background doesn't cover the entire screen

After I updates to plasma 6 my login background is not covering the entire screen, leaving white borders at the bottom and to the right (see image). It worked correctly on plasma 5.

The image itself is 4K res, but I also tried scaling it down to 1440p. It didn’t make any difference. I was also unable to find any scaling options in KDE system settings.

Am I missing something obvious here? I would be very grateful for any help.