Ubuntu Server 20.04 did do an upgrade to 20.10 without explicit indication
Ubuntu 20.04 LTS is supported until 2025 for free, then for 2030 with paid ESM.
Having Prompt=normal on LTS is bad idea. Currently 20.10 is EOL.
To prevent upgrades from LTS to non-LTS versions you have to edit /etc/update-manager/release-upgrades and set Prompt=lts here by using any text-editor like nano.
For release list see https://wiki.ubuntu.com/Releases and https://ubuntu.com/about/release-cycle .