Why does my MacBook spazz out, but not when connected to external Dis?

Hello everyone,

I have a 2015 MacBook Pro Retina that I purchased new from Apple. It has been taken great care of through its life and kept in a protective case when it’s not in use. I replaced the battery a few months ago due to minor swelling of the original battery and the laptop has worked fine ever since. One morning I opened the laptop to be surprised with a display that was spazzing out with colors everywhere. I connected the computer to an external display to check to see if the GPU was fried, and to my surprise, the display quit spazzing out and worked completely normal. When I unplugged the external display, it went back to spazzing out.

I erased the SSD and reinstalled macOS thinking that would solve the problem. Nope

I reset the SMC and the NVRAM. Nothing.

This confirmed three things:

1. The OS is not the issue
2. The built-in display works fine when connected to an external display.
3. The GPU is operational.

Does anyone know why my MacBook is having these issues?

Please watch the included video of the laptop connected to the external display.

Thanks!

J