Hi, after installation of kde neon, distro won't appear in grub boot menu, so i used another distro to update grub via update-grub and everything worked.

Best of luck.

Also, after kernel update, new kernel won't appear in grub.

I guess i have 2 grub bootloaders installed.

If you have multiple distros installed, each one has its own grub.
Which one is showing when you boot is based on which one is set in your bios.

I installed grub to mbr via grub custimizer, and it solved ubuntu mixed grub problem