Check the hardware settings in CoreELEC and try changing the VESA modes settings, it can make a difference as my older box worked better with it on, my N2+ works better with it off, but its dependant on the TV hardware so its experiment time.
If your on Kodi Leia then the FAQ for the CastagnalT git hub repository says this is a known issue that’s down to Widevine security. However if your on Kodi Matrix (testbuilds) then the Latest Version (v1.13.0) does say its fixed. Though I haven’t tried it as my other half is watching the Christmas episode of “Call the Midwife” and I like my head where it is.