More explanation: they tested a monitor in an environmental chamber at -40 C and it started showing red ghost banding exactly like my phone has been doing since the screen replacement. Their explanation was that the driver was having trouble driving the crystals hard enough to twist them under those temperatures.

This made me think back to the correlation I'd noticed where my screen exhibited more banding issues when the phone was stored in a warm area (even if the screen was turned off!), and the replacement screen was missing a foam pad on the back that the original screen did have.

I think the pad was meant to insulate the screen against heat from the SoC to prevent it from getting warm enough to cause driver issues? And now that protection isn't there, and so whenever the phone is warm, it's causing the screen to have issues.