Azure Custom deployment issues with keyvault parameters

Hey Team,

Just hit an issue in Azure portal, I can't find any information about online. I have started working with Arm templates after using Terraform for the last few years. In my learnings, I am using the Azure Portal service "Deploy from a custom template". I have created a template from a deployment and updated it to use a keyvault for the password, following this doco Key Vault secret with template - Azure Resource Manager | Microsoft Docs

Now my issue is that when using the Azure Portal service "Deploy from a custom template" and uploading the parameters file the password parameter reverts to a null value. This then stops you from deploying,

If I deploy the same template and parameter files via Powershell it works as intended so not sure if this is by design or not.

Anyone have info on this?