Implement interface only working after specifying import

[RollingSquirrel]

Does this issue occur when all extensions are disabled?: Yes

• VS Code Version: 1.58.2
• OS Version: Windows_NT x64 10.0.19042

Description

I am developing an express.js application with TypeScript.
The "Implement interface" usually available with shortcut or "Quick Fix" stopped working in my project over time.
This seems very odd to me since I did not change anything in the tsconfig.json during this time.

I have tried to fix the problem with the following:

• Delete tsconfig completely (and restart after this)
• Disable all extensions
• Restart (Pc & VSC)
• Restart ts server (command in VSC)
• Reload project (command in VSC)
• npm ci, manually remove node_modules
• Restore repo to the last time this option worked (hard reset, restart)

After playing around with the code for a while, I discovered that importing the node_modules dependency used in the type declarations of the interface, makes the implement feature available again.
I have tested other imported types (e.g. typeorm) which seem to work fine without specifying the import.

Steps to Reproduce:

I will provide a simple example that requires the Router type of express.js.
This worked fine a few days ago and stopped working after some time. I attempted to reset everything but it did not change the behavior. Is there any cache / something similar that I should reset / delete?

controller.ts

import { Router } from "express";

export interface Controller {
  getRouter(): Router
}

user-controller.ts

import { Controller } from "./controller";

export class UserController implements Controller {
  // want to use "implement" here
}

Depending on the import the option appears again:
Without import:

With import:

Let me know if you require more details.