DrDiettrich:
If these are really simple LEDs, you need an adjustable current source. A FET or transistor by itself, driven by PWM, will not act as a current source.
Have a closer look. OP is using a mains powered 300mA constant current source.
With a string of 10 LEDs (should be 11 or more for this supply).
The supply is likely to have a fit during PWM-off periods.
OP should have used a constant voltage supply, e.g. 12 or 24volt, and PWM-able constant current drivers.
Leo..