When switching screens have to drain the battery

This is an interesting one. Have a iPod Touch 4 / 32G on the work bench. I put on an old cracked test screen that I use for diagnostics. However, whether I put on a test screen or new screen I have to completely let the battery drain, otherwise the display will stay blacked out. Once drained, connect up the charger, and the display comes on.

Really can't see if doing a Power+Home reset will correct it, since I haven't soldered on the new flex cable yet (ripped power button connection).

I've swapped screens before without having to power down and have never experienced this particular "gotcha" :)

Just curious to hear from other techs.