@frxnz consider powering the nano and the neopixels separately. IE go RPI + Nano on one supply, NPs on another. The reason for this is that you're going to get a voltage sag when the NPs start drawing current. Specifically when you do a big srtrip update and drag a lot of current, you'll see a V dip everywhere else on the nano.... this might be part of what you're seeing because if there's a dip it might not make the logic levels on the I2C bus. I'd try that and see what happens.