null check of a const property incorrectly resolved

TypeScript Version: 3.5.3 (also tried with @next)

Search Terms: const symbol property "not assignable to type"

Code

/*
 * For reasons of how the JSON is converted, I'm using Symbols to hide certain properties.
 */

class Server {
    public auth: string | null = null;
}

/** `tsc` complains about this[AUTH_PROP] even though it must always be a string inside the `if()` */
const AUTH_PROP = Symbol();
class TestSymbol {
  private readonly [AUTH_PROP]: string | null = null;
  public readonly server: Server | null = null;

  public get auth(): string | undefined {
    if (this[AUTH_PROP] !== null) {
      return this[AUTH_PROP];
    }
    if (this.server && this.server.auth) {
      return this.server.auth;
    }
    return undefined;
  }
}

/** same thing with a string literal and it works */
class TestNoSymbol {
    private readonly _auth: string | null = null;
    public readonly server: Server | null = null;

  public get auth(): string | undefined {
    if (this['_auth'] !== null) {
      return this['_auth'];
    }
    if (this.server && this.server.auth) {
      return this.server.auth;
    }
    return undefined;
  }
}

/** same thing with a const string and it fails */
const AUTH_PROP_S = '_auth';
class TestConstString {
    private readonly [AUTH_PROP_S]: string | null = null;
    public readonly server: Server | null = null;

  public get auth(): string | undefined {
    if (this[AUTH_PROP_S] !== null) {
      return this[AUTH_PROP_S];
    }
    if (this.server && this.server.auth) {
      return this.server.auth;
    }
    return undefined;
  }
}

Expected behavior:
All 3 versions of this should be fine, and not throw a tsc error. The const values can never be changed so it must always be a string inside the if() statement.

Actual behavior:

Type 'string | null' is not assignable to type 'string | undefined'.
  Type 'null' is not assignable to type 'string | undefined'.

Playground Link: https://is.gd/V7Jai9

Related Issues: not sure