Hi, please help, after power on TV and my Mecool kiii pro with coreelec, kodi is in 480p. After reboot box with tv power on, kodi boot in normal resulution 4K. Why kodi in first boot change resulution to 480p? How fix it?
Never saw such small resolution on startup
What CE version do you use? Did you see 480p video before box shutdown?
Try to power on TV before box power on
8.90.5 on sd card, no none 480p, if tv power on first, kodi resulution is correct.
But i have extension cord and everything connected into it.
I turn on extension cord with sonoff and box turn on as the first one.
Box is connected to av receiver Marantz.
If I remove sd card with CE, box boot to the original android and resulution is correct.
The bug is in CE or in settings CE(kodi) but i do not know where.
I try off CEC, off hdmi control in av, did not help.
In one of the past few builds we removed the bottom limit Kodi had on resolutions.
In your case, EDID is probably not available when the box is booted up before the TV does, and it defaults to the lowest resolution possible.
Read here for a solution: (reboot your box with the TV on first)
unfortunately, it dit not help 
But If i set 1080p its ok after power on at once. 2160p not.
It’s been repeated, many times, that is not recommended to set the GUI above 1080p. The player will switch to a higher (or lower) resolution as required.