@Pryde: Sorry to hear that you're having issues with your S Pen. Can you try placing it back in its holster so it clicks into place, then press the Volume Down and Power buttons together until the phone restarts. If the issue persists, head to Settings > Advanced Features > S Pen > Air Actions > Tap the 3 dots in the top right > Reset S Pen, to see if this helps. Let me know how you get on.
This is also happening to my second Note 20 Ultra. I tried reseting it while putting my verified to work pen from my other Note, which works with both the pen from the working Note and the non-working Note. Also didn't work. Even when the working note had a pen that wasn't connected to it inserted and taken out the phone would vibrate and act like normal as when a pen is taken out. The non-working Note doesn't react to it going in or out.