Passing secrets with decorator @secure() shouldn't give an error

Bicep has a decorator @secure() that makes an output be able to pass secrets.
The problem is even using it, I still get an error from ARM toolkit.
Here is the official documentation: https://learn.microsoft.com/en-us/azure/azure-resource-manager/bicep/outputs?tabs=azure-powershell#secure-outputs
Here is the line of code I'm using (Bicep):
```
@secure()
output accountKey string = storageAccount.listKeys().keys[0].value
```
Output in JSON:
```
"accountKey": {
"type": "securestring",
"value": "[listKeys('storageAccount', '2025-01-01').keys[0].value]"
}`
```

Here is the error:
```
NestedTemplate [parameters('storageAccountName')] [ Lines 485 - 776 ]
[-] Outputs Must Not Contain Secrets (6 ms)
Output contains secret: accountKey
```
Since the type is securestring, shouldn't the toolkit consider this as not an issue?

Start with the ARM JSON generated from the shown Bicep and the ARM-TTK “Outputs Must Not Contain Secrets” check; trace how securestring outputs are classified. Compare that behavior with the linked Azure secure-outputs documentation, then add a regression case showing that @secure() outputs pass while non-secure secret outputs remain rejected.

Written by the indexing model from the issue text.