Macbook screen flickering issue

I’m currently using MacBook Pro 16inch.

I usually use MacBook with LG ultrafine display, with my MacBook in clamshell mode. Also, when using Bootcamp, I use NDVIA eGPU.

Here’s the problem.

Recently, after using Bootcamp windows, I ejected both eGPU and Ultrafine display. After that, I started to get a flickering issue with windows update screen remaining as a ghost.

To solve the problem, I did every kind of reset, including factory reset and NVRAM, SMC reset. But the problem remains.

Strange thing is, if I connect my MacBook to any external display, these issues does not happen. These problem only happens when I use my MacBook alone.

Even in repair center thought it is strange, and tried installing new main board. But the issues remain.

Since the internal display works perfectly when MacBook is connected to second screen to operate dual screen, this issue doesn’t seem to be display’s problem.

Does anyone have any idea about what is possibly causing this problem??