OK. Unfortunately I don't have a good idea of what might be causing this problem, but I will suggest that you try the 2.0.0-beta.11 version of the beta-phase Arduino IDE 2.x. This version has a new firmware updater tool that uses the most modern software. You can download it here:
https://www.arduino.cc/en/software#experimental-software
This is primarily intended for beta testing, so you might prefer to go back to your stable Arduino IDE 1.8.15 for daily use. But if it gets your firmware updated that's a good thing.
If you like command line tools, you can use the Arduino Firmware Uploader instead:
https://arduino.github.io/arduino-fwuploader/latest/
This is the tool used under the hood by Arduino IDE 2.x, which just puts a nice GUI over it, so there is no difference between using the IDE or using the tool from the command line other than the GUI being convenient.