Why does my device keep going to sleep and back very frequently?

I have bought a used Macbook pro 13 inch early 2015. After about a month of usage, the screen started dimming down and up very frequently. Around once per second.

Before this incident I have not used the laptop for about 3 days.

The battery is fairly new from ifixit.

I had installed AlDente software at some point prior to this to maintain my battery life.

Since then:

• The keyboard only works when the screen is lit.
• The trackpad only works when the screen is lit.
• I was running catalina. I have successfully formatted the SSD and installed Monterey (trying to type as quickly as I can when the screen is lit)
• I have also opened the laptop and disconnected the battery, then connected the laptop directly with the charger, started it and the issue was still there
• ONLY when I connected to an external monitor, for some time it worked fine, and then stopped. It could not detect the camera when it was connected to the external monitor. (The lid was open)

I have atteched couple of images to show the issue.

Could you please help? I am not sure what is broken or what needs to be fixed.