The Pi will by default boot in the native resolution reported by your TV using EDID.
So if your TV reports 720p as its preferred resolution that’s what it will boot in. This default resolution is reported as “DESKTOP” in Kodi.
If you change the resolution in Kodi then Kodi will use that new resolution however the early boot process including splash screen will still use the preferred resolution reported by the TV. This is normal and the way the Pi works.
If it bothers you you can override the boot resolution of the Pi by using config.txt options - check out the resolution settings in the Pi settings module in OSMC settings, or refer to the following table of hdmi_mode and hdmi_group settings: