Got it, Mabujaro,
Were you able to hear out of both AirPods when you adjust the balance?
If you create a new user account to test your AirPods, do you experience the same behavior?
To create a new user account, you can go to the Apple menu > System Preferences, then click Users & Groups.
Click the lock icon, then enter an administrator name and password.
Click the add button (+) below the list of users.
Complete the fields shown for an Administrator account, then click Create Account (or OK).
Once you’ve created the new account, go to the Apple menu > Log Out, then log in with the new account to test.
If you're unable to reproduce the same behavior in a new user account, you may consider troubleshooting your login items as outlined here: Remove login items to resolve startup problems on your Mac
If you're still unable to isolate the behavior of your AirPods while connected to your Mac, we'd also suggest that you reach out to Apple directly here: Contact - How to Contact Us - Apple
Kind regards.