[FXB78]

You mean you are not using Ugoos stock firmware?

I'm using stock Ugoos dual-boot firmware, it's the only way to use CoreELEC. But I'm using OTA updates for Android rather than flashing the updates, because I think doing that will reset CoreELEC to an old version.

That means... you can update only Android through official OTA? How you are doing this? please elaborate...
Can the updates on dualboot CoreELEC done separately?

If you flash the firmware, I would expect it to flash CoreELEC back to whichever version Ugoos are using, but I never tried it. I flashed it once then used OTA updates. With CoreELEC you can use OTA or drop the update in the update folder and it will manually update.

You can update Android OTA via the GUI.