Yes notifications work with stock mail and the FastMail app.Another couple of things you could try to get at least a little more info that might help you:
(1.) Add the FM account to Accounts and Passwords and see whether you can get notifications in the stock Mail app.
(2.) Download the FastMail client, which, obviously, should have no trouble with it.
If you can get FastMail notifications with one or both of those methods, but not with Spark, you'll at least know it's a Spark issue rather than a FastMail issue. I think.