Being currently in contact with support, it seems that the problem originates from using Fine-Grained Password Policies (FGPP) in conjunction with the type of querying the password expiration information used by LanSweeper.
I found this hint on the web (https://learn.microsoft.com/en-us/answers/questions/110116/active-directory):
When using "net user samAccountName /domain", the value returned by "Password expires" doesn’t take in consideration the fine grained policies.
It only shows the domain password policy.
This might explain, why LanSweeper currently shows a wrong expiry information, when FGPP is not taken into account.