Rxjs of operator doesn't emit value when using proxies

I have a simple unit test that uses Substitute.js (also tried this with TypeMoq mocks and the behavior I'll describe is the same).

In this test I try the simplest thing, using the of operator to emit the mocked object. Without any further operators, the subscribe callback is never called. Example :

import {Arg, Substitute, SubstituteOf} from "@fluffy-spoon/substitute";
import "reflect-metadata";
import {Observable, of} from "rxjs";

const factory = Substitute.for<MessageFactory>();
of(factory).subscribe((f) => console.log("got it")); 

The console log is never called.

Now, if I don't use the operator and I just create an observable, the log is working. Example :

import {Arg, Substitute, SubstituteOf} from "@fluffy-spoon/substitute";
import "reflect-metadata";
import {Observable, of} from "rxjs";

const factory = Substitute.for<MessageFactory>();
new Observable((subscriber) => {
    subscriber.next(factory);
    subscriber.complete();
}).subscribe((f) => console.log("got it"));

Any clues on what's going on with the of operator in this case?

I'm using :

• rxjs: 6.5.4
• typescript: 3.5.3
• node : v10.18.1