I’ve seen some glitching with even as few as 24 pixels. It appears that the neopixels are more finicky about their power supply and signal voltages than the spec says. Adding capacitors along the tape may help.
We have tried reverting to 27 bytes per pixel, but didn’t see a difference in performance. I haven’t totally figured this one out yet, but you’re doing all the things I usually suggest, short of adding caps.