Yes it's normal that your invoice isn't automatically updated when you change something at the client level. That's because invoices and clients are two different things in our database and because it's too risky to edit the data of an invoice when it has already been sent to a client.
Imagine you change the address of a client, it would mean that all the invoices you ever created for this client will be automatically updated (even those from a long time ago!).
Changes at the client level thus only apply to the new documents you create. If you want to apply a change to an existing invoice, you have to delete it and recreate it.