Hi, thanks for your reply!
Unfortunately, I already tried your idea and it didn't work...
I have two admin accounts, (1) the new one I want to use b******************.com and (2) the built-in Administrator account I activated temporarily. The account I've been using for a long time (the one I want to change to a Standard User type) is u*********.pl
Here's the cmd log for net localgroup with the commands you recommended:
C:\Users\b****>net users
User accounts for \H*****
Admin Administrator Gość
Konto domyślne postgres u*********.pl
WDAGUtilityAccount
The command completed successfully.
C:\Users\b****>net localgroup administratorzy # (administrators)
Alias name administratorzy
Comment Administratorzy mają pełny i nieograniczony dostęp do komputera/domeny
Members
Admin
Administrator
LOCAL
MicrosoftAccount\b******************.com
NT AUTHORITY\SERVICE
The command completed successfully.
C:\Users\b****>net localgroup użytkownicy # (users)
Alias name użytkownicy
Comment Użytkownicy nie mogą przeprowadzać przypadkowych ani celowych zmian na poziomie całego systemu i mogą uruchamiać większość aplikacji.
Members
MicrosoftAccount\u*********.pl
NT AUTHORITY\Authenticated Users
NT AUTHORITY\INTERACTIVE
u*********.pl
The command completed successfully.
As you can see, everything looks as it should.
Ever since I changed the Account type in GUI, from the b******************.com's side the u*********.pl's type is Standard User:

I logged off and logged into u*********.pl
Win+R -> netplwiz:

It seems all right, doesn't it? Let's see Account Settings from u*********.pl's side:

Administrator... Plus, this account really does have administrative privileges.
So - how can I fix this? Any ideas?
Thanks in advance,
BM.
PS: Some things are in Polish (it was the default language, but I changed it to US English), so:
Użytkownicy is Users
Administratorzy is Administrators
Gość is Guest
Konto domyślne is Default Account.