Flyback simulation - not regulating at lower load
Flyback regulators are power regulators that give the impression of voltage regulation when used within a closed loop that can alter the duty cycle. With your lower limit at 5% you might hit problems at light loads as you are seeing.


  The output waveform I've shown for 0.1A has a gate signal for Q1 with
  5% duty cycle.


In the power transfer, you charge an inductor (P1||L2) with energy and that energy is proportional to duty cycle squared. This is because the current is proportional to duty cycle (\$V\cdot t/L = i\$) and stored energy is proportional to current squared (\$W = \frac{Li^2}{2}\$).

Given that your maximum power is 100 watts and your minimum power is 1 watt, you might need to rethink what the PWM limits are if you are using 50% duty for the maximum power scenario. At lower duties the core may not be saturating as muchs and this would increase the power transferred. I would suggest you try and get down to 1% duty cycle and see what happens.

If maximum power is acheived at 25% duty then you would need to get below 2.5% duty to achieve voltage regulation with a 1 watt load and probably much less if you are close to core saturation on higher powers.