Why does Ubuntu 20.04 update reset my BIOS to boot 16.04?

I am dual booting Ubuntu 16.04 and 20.04. I don't use 16.04 much anymore and its just there because I haven't got around to removing it.

Every time Ubuntu 20.04 updates, it resets my BIOS boot settings to boot 16.04 by default. Why does it do it and how to prevent it?

EDIT: More details:

1. Booting in uEFI mode (secure boot disabled)
2. Asrock Z390 Taichi motherboard with Intel core-i5 9400F CPU (no integrated graphics)
3. RTX-2070 Rev. A GPU with driver: nvidia-driver-450 (proprietary, tested)