I don't know 100 % what the issue is but If i had to guess i highly dout hte Light system produced RF . I do suspect that the system has power requirment that maybe your on bord system is haveing a issue meeting

sepcily if your useing switching ESC with built in BEC .. normaly these are really current limtted . please check your specs and give us the BEC power specs

if you'd like a quick troubling shooting tool plug in a stand alone BEC , or just a bench testing battery piggy back it into the the RX's power buss then have friend hold airplane and repeat your range test issue

thats my thoughts that the light system might be puting extra load on the BEC that was already close to maxed out so your over power could be closer to brown out stage .. granted 72 MHZ can run at VARY low voltages but i suspect if your power goes down to sub 4 volts issue it might still work but might be prone to some glitches .

good luck