iPhone turns on right after turning off

So I recently fixed a water damaged iPhone, fairly simple repair, just needed a new battery, screen and to reflow one of the touch ic chips (I know it is a bad practise, but I got this cheap damaged iphone just to get some experience on microsoldering and repairs in general).

The thing is, I have a quite wierd problem. Whenever I power off the iPhone, it turns off and right awat i turns back on again. It happens aswell when the battery is discharged, it keeps showing me the logo with the battery and the lightning cable.

The power butron is not faulty since the phone doesn’t lock or turns off by itself.

Can anyone give me a clue to why this is happening?

Thank you!