I think you are referring to the startup phoenomenon. Yes, there needs to a positive feedback loop in order to startup this loop. during startup, when the system is slightly perturbed and when you have a small current flowing thru the transistors, gm is also very low. At this point, you'll have a loop gain of close to 1 (according to the equation, if gm ~ 0, then loop gain ~ 1) In such a case, the bias circuit is regenerative and node voltages will rise because of the positive feedback. However, when the nodes voltages have started to rise, the loop will become less and less regenerative because gm8 is rising causing the oscillations to die down.
I misunderstood your question at first. I thought you said you were seeing oscillation after startup. And that is why I told you that it could not occur if devices are sized according to the equation because the loop gain is less than 1. However, during startup this is not the case.