This sounds like a power and/or ground signal related issue that is creeping in when power draw increases. It doesn’t sound like an issue with your code.
8M is quite a long distance for this type of LED data signal. I suspect that once power draw really kicks in, the difference between the signal ground and your power ground becomes higher, and is pushing it outside of operating margins. You might get it to work better with improved ground wiring.
For distances this long, I would add in some differential transceivers, like RS422 / RS485.
If feasible for you, another working option would be to move the expander closer to the LEDs and run a long expander data line instead. Its protocol will handle communication errors much better. It can also be extended with a differential transceiver.