This happened to me on Ubuntu 20.10 using latest kernel 5.8
Lenovo Ideapad keyboard
Important: previously I had set a keyboard key activated by AltGr (right Alt) + another key. Apparently, AltGr brings all kinds of weird characters, avoid using it.
Two possible solutions for it:
Go into Accessibility Options, enable and disable Sticky Keys.
This brought my "s" key back, but the problem came back after a while.
Go and edit that keyboard hotkey you added before, use anything else other than AltGr (never use this key) In this case, I re-assigned the same hotkey to RightShift+Keypad8
After doing this, my "s" "-" and "+" keys were back.
In your specific case there, as it is a fresh install, try manually reseting all keyboard hotkeys, or change them to something else as a test.
If none of this works, use terminal command xev and see what happens when you press a key that works, and see what happens when you press the problematic key.
If the problematic key causes a two-line array full of 0 0 0 0 0 0 (zeros) to appear, then it is being treated as function key for whatever reason.
I've noticed that right when I boot into Linux and I press the Fn keys on my laptop there's a message that appears saying "Sticky Keys Enabled". I'm not sure if that's the correct functionality of it. Might be a bug.