@Vero one more update, maybe it could clarify something.
I've just tried to set Spark 2.5.1.668 as a default Mail app for Catalina (10.15.2). When I tried to email the file using the Action (Email to...) action from Alfred, Alfred opened the Mail.app instead of Spark. Even though Spark is set a a default app for Mail. Not sure if that's related but seems like a similar kind of an issue.