Quick update: last night I installed the original GameFrame sketch onto an Uno and tested the same hardware and wiring – worked perfectly. Same SD card, card reader, wiring, lights and other components. So I’m now more confident that the problem I’m having is related to running the V2 code with the ws2812 lights (or possibly related to running a teensy 3.2 or 3.6 rather than the LC).
I don’t understand the differences between FastLED and Adafruit’s neopixel library well enough to troubleshoot the code, but I did notice that the V1 code buffers 1 pixel (3 leds) whereas the v2 code buffers 16.
Jeremy, do you happen to recall, did you use the smaller buffer with v1 because of a hardware limitation in the Uno or was it related to the ws2812 lights themselves? It’s probably unrelated to my problem, but I’ll mess with this tonight and see if it helps the glitching problem.
Any other ideas are welcome.