I am experiencing some VERY wonky behavior with my Pi and breadboard with the LCD screen https://www.amazon.com/gp/product/B07FK8GB8T

I am THOROUGHLY confused. The code WORKS (with all the bugs present of it) when I use the rails from the raspberry pi for power. Both 5V and 3.3V works, but this is not optimal. I'd like to be able to use an external power source to power the LCD and there really shouldn't be a reason why the power would be tied to the I2C communications, right? Could anyone explain this?