Validation rule for postal code is being ignored

In the SharePoint list, there is a column “Country” and a column “Postal Code.” A validation rule has been configured in the list validation settings to ensure that when the country “Germany” is selected, only postal codes with exactly 5 digits can be saved.

Currently, this validation rule is being completely ignored. Regardless of whether the postal code contains fewer, more, or no digits at all, the entry can always be saved. The validation does not take effect.

Expected Behavior:
When Germany is selected in the Country field, the entry should only be saved if the postal code contains exactly 5 numeric characters.
If the input is invalid, an appropriate error message should appear and prevent saving.

Actual Behavior:
The validation rule is ignored.
All inputs — including invalid postal codes — are saved.
No error message is displayed.

Impact:
Incorrect or incomplete data is stored in the list.
Manual corrections are required afterward, causing additional effort and increasing the risk of errors.

The english version of formule we used =IF([Land]="Deutschland", IF(LEN([Postleitzahl])=5, 1, 0))

Why doesn't the validation work? Thank you in advance for your comments !