One part of the video stack has decided to render 1080p, another has decided to use a 4K mode so you're looking at 1080p rendered onto a 4K surface. I'm not sure what the root cause is, but this is occasionally reported against x86_64 and RPi hardware so it's likely a logic failure somewhere in the kernel DRM code rather than being something vendor specific.
Assuming HDMI is on the HDMI-A-1 connector, append video=HDMI-A-1:1920x1080M@60D to kernel boot params in syslinux.cfg or the equivalent EFI config (depending on which is used for boot). This will force the initial kernel DRM state to 1080@60, which will stop any attempts to use 4K modes during boot. It doesn't affect Kodi switching to 4K modes for playback.