windows 10 - user does not have access to folder even if he is in a group with full control

I figured out a super secure (ransomware proof) backup solution using free Syncrify backup software. It looks like it is working BUT now when I want to check something if it is in that folder I runas cmd as the user backuper but can't access it!

D:\zaloha>cd Syncrify
Přístup byl odepřen.
D:\zaloha>whoami
athena\backuper
D:\zaloha>cacls syncrify
D:\zaloha\Syncrify BUILTIN\Backup Operators:(OI)(CI)F
D:\zaloha>net localgroup "backup operators"
Název aliasu     backup operators
Komentář         Člen skupiny Backup Operators může pro účely zálohování a obnovování dat překonat zabezpečující omezení.
Členové
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backuper
Příkaz byl úspěšně dokončen.

So why the heck can't I access the folder when running cmd asi backuper user which is member of Backup Operators group which has full control for this folder?? Then I tried to add specifically backuper user to this folder permissions and suddenly I can access it. Is this group somehow broken on my Windows 10 machine? The syncrify service runs under backuper user and happily writes data to this folder. Why can't I access it using cmd is a mystery for me.