The reason beeing that Azure CLI and Azure RM commands are context sensitive in regards to subscriptions. If for some reason you have Azure Accounts with access to more than one subscription you might be unable to use scripting to manage those resources, since the Azure Acccount's default subscriptionId might not match the resource's subscriptionId.

My suggestion would be to just add a new field/setting/variable for all Azure resources that Octopus stores information about, where you already take care of the ResourceGroupName and treat it exactly the same.
Azure provides the information in the same context at the same level, and it is one of the few default parameters that are present for 99% of Azure scriptable commands.

Background for the feedback:
https://help.octopus.com/t/octopus-action-azure-subscriptionid-seems-to-default-to-the-wrong-subscription/27806

Note that I am not suggesting that you change the Octopus.Action.Azure.SubscriptionId from the account (as it is today), merely that you add an additionla Octopus.Action.Azure.WebAppSubscriptionId or similar since it might not be the same value all the time.