It is a sign of heat issue ( not your case as your temps are good ) , can be from the light too close ( your light is 40 cm away but they are both 54w so I feel like the distance is good ) , can be from your watering routine aka underwatering , can be due to the too high or too low humidity level which is a bit too high for that stage as the max value is more around 70% ( between 40% and 70% is good for that stage ) so you'll maybe have to low your humidity level generally it's a air intake and outake issue