Avatars from 2 Accounts unwillingly linked by setting an iCloud avatar

I had 2 Different accounts on my mac for different working puposes: For me it's very important to completely divide these scopes into different users. The 2 users share the same iCloud account to synchronize Calendar and Notes. They have different avatar pictures.

Then I chose an avatar picture for my iCloud account. This caused that the avatar from both accounts were changed to that iCloud avatar. Now: If I change one avatar, the other one is changed too. It's not possible anymore to have different avatar pictures for the 2 accounts. I guess this is because they are both linked to the iCloud avatar.

I spent a lot of time on solving this problem, but there doesn't seem to be a solution for this bug.