CO2 laser gets weaker above 50%

My 40-50W co2 laser cuts 1/4" plywood and acrylic well at 50% but above 50%, the cuts get shallower as I increase the power. The current stays approximately proportional to the power setting, but the beam seems to get weeker. Is this normal or a sign of a partucular failure?
The tube has less than 100 cutting hours, mostly at about 45%. I almost never run above 15mA current.
The tube is a TONGLI 40-50W Co2 Laser Tube
35-50W CO2 MYJG Serise Power Supply MYJG-40 OG 110/220V × 1