Using a connection string to connect to Blob storage

Hi

This might be a simple one but I've searched around a lot for an answer on this and have come up with nothing so i'm hoping someone here can help. In Synapse I just want to create a linked service to Blob storage using it's connection string stored in Key Vault - it allows me to create it and 'test connection' succeeds but when I move to the Data tab in Synapse and try to view it there it show an error

It seems to think the account name is blank, note there is nothing in the parenthesis: 

I also get this error when i hover over the red X:

This does work when i enter the storage details manually and use key vault to get the access key, rather than the whole connection string so it's definitely not an issue with permissions on the key vault.  I wondered the  connection string needs to be in any particular format to work - I just copied it directly from the storage account in the portal and it looks correct. I've tried this with different storage accounts on different tenants and its the same. What am I missing?

Thanks :-)