Thank you for reporting, I could indeed reproduce the issue using xLights!
After 2hrs of troubleshooting, managed to fix the issue. It was an off-by-one error that actually caused the last LED of the first universe (170th) to stay unlit.
The strange part is that this didn’t happen with LedFX (my go-to E1.31 testing client for its simplicity), despite it being set up for 510 channels as well.
In any case, the issue is fixed now in latest master!