Custom Prevention Policy only using application rules is not applied to Asset, even when it shows the policy as applied.
If you want to use a custom prevention policy, the server expects a custom sandbox to be in the custom prevention policy.
You can add Application routing rules directly to the base policy.
To use a custom prevention policy, the server expects a custom sandbox to be in the custom prevention policy.
Even if you do not have a specific sandbox required for those application rules, you’ll need to add a dummy sandbox.
You don't need to actually use the custom sandbox, but you’ll need to have it there for the server to use your custom prevention policy.