I've had problems in the past where the property style (vs. method style) object type definition prevents class instances from satisfying the type. The reason is that flow requires the properties to be own props, but class methods are inherited (on the prototype).

There's some related info here, here, and here

One way to verify is to create a class that has a next method and see if it type checks as a Subscriber. Something like:

class Test {
  next (a: number) void {}
}

const s: Subscriber<number> = new Test()

If that doesn't work, I think one solution may be to make the properties covariant explicitly:

export type Subscriber<A> = {
  +next?: (value: A) => void;
  +error?: (err: Error) => void;
  // complete value parameter is deprecated
  +complete?: (value?: A) => void;
}

Nice