4.3" touch TFT needs USB + external power?

Hello all. I have an Arduino Mega with a SainSmart TFT shield and 4.3" touchscreen TFT (SSD1963). The Arduino takes 12V through the DC jack via a 12V 8A wall transformer (which powers other accessories, namely 5630 LED strips, hence the high current rating).

When only USB is connected to the Mega, the screen displays fine. When only 12V is plugged in, the screen is all white as if it's not receiving enough power. As such, to get the TFT to function properly, I need both 12V and USB plugged in. Is that normal? Is the USB somehow "evening out" the voltage in?

I can post my Fritzing design if necessary, but it's kind of a mess right now. I can try my best to clean it up if needed, but if this is a common enough issue that doesn't require it, I won't bother. Thanks in advance!

Update: I fixed the problem by using a 12V to 7.5V buck converter to power the Mega. Apparently that's what it took to "smooth out" the voltage in.