Hey certainly_heisenberg,
It appears that you're having a problem with the Reminders app on your MacBook Pro since upgrading to macOS Big Sur. When you select "Clear All", only one of the Reminders clear, and we're glad to provide assistance.
In order to determine what may be causing this, we have a couple of steps to isolate the issue:
How to use safe mode on your Mac - Apple Support -- We'd like you to restart your Mac in safe mode and test the issue there. Be aware, "When you start up in safe mode, your Mac prevents some software, such as startup items, from loading, and it performs a check of your startup disk. Your Mac may take longer to start up because of the check." To exit safe mode, restart your Mac normally.
New user or group setup - Apple Support -- If the issue happens while in safe mode, create a new admin user account, then set up a few reminders to test.
Please let us know the results of both tests when you're able.
Take care.