Two possible reasons:
1. Due to Jenkins' bug, we cannot get value of previous radio block, so I don't know you're choosing Create new Resource Group or Use existing Resource Group. The workaround in the plugin is guessing by whether you have typed something in Create new Resource Group. So if there is something in Create new Resource Group, the Use existing storage account will always take content in Create new as dependency. If you're using Use existing Resource Group, please make sure to clean content in Create new Resource Group.
2.
Use existing storage account will match your type of storage type, so please make sure the storage account is either Standard_LRS or Premium_LRS.

zackliu [1] indeed seemed to help. However, in addition to that I had to select Use Existing under Storage Account Name then change Resource Group Name to get contents reloaded.

Sorry I dont understand what you mean, we cannot add an existing resource group and if we leave it empty it just enters a random name after saving. How can we use an existing resource group?

shepyard You need to do some change in storageAccountType or ResourceGroupName to let Jenkins UI to reload the existing storage account.  Or you can save your configuration and reopen the global configuration, then the account will also be reloaded.

hey zackliu , kentzo azure_devops

i am still experiencing this issue with the latest plugin

trying to use a private storage in Azure, and the list is empty 

using 

Thanks