Belts wise, your test is perfect
Your Gcode from lightburn seems perfect
2000mm/min, S600 max
I wonder if you can export Gcode when using Neje software?
Something dont add up really
or maybe show us your full screen of lightburn?
Plus all @berainlb said 
I keep getting hung on this from the first post. Is there a chance the firmware has two ātablesā and itās not switching correctly and/or not reporting to LB properly? Would a reset be beneficial?
My Ikier does some strange things with its firmware and it took me a while to figure out. LB doesnāt account for it. I have to either manually edit the āproblemā $ values or handle mode switching offline from the laser controller screen itself. I wonder if Neje is doing something ācreativeāā¦
Unless Neje have changed philosophies on this very recently there should be only two states to the firmware: proprietary mode and GRBL mode. The machine is certainly running in GRBL mode based on output.
I do recall Neje having issue with power output in the past with certain firmware which is why I suspect it may be a firmware issue fundamentally.