I’m not an HCP either, but there’s fluttering and there’s feedback, which are 2 different things. Feedback can either be a constant pitch sound or a whistling sound, depending on the nature of the feedback. But fluttering is more of a wobbling sound.
I think what you’re describing (over extending in the high) is probably more likely feedback than fluttering. But actually a stronger aid usually would increase amplification and cause the feedback to be worse, not better, if the problem is indeed feedback. After all, it is the stronger amplification that is causing the feedback back into the mics in the first place.
Usually one of the traditional feedback strategies is to slightly shift the frequency by 10 Hz or so. The idea I think is to disrupt any potential resonance that would promote feedback. Usually, most people can’t tell a difference in the overall sound if the frequency is shifted by only 10 Hz before amplification. But if a pure tone is picked up by the mics, shifting it by 10 Hz then amplifying the sound, and hearing the natural unamplified sound through the vent in parallel, has the potential to create the fluttering effect because you’re not hearing just 1 pure tone anymore, but now you’re really hearing 2 pure tones, the unaided original pure tones, and the amplified pure tone that’s 10 Hz off, hence the fluttering effect.
For the Oticon OPN S and the More, there’s another effect due to the feedback prevention technology (called the Optimizer) that Oticon employs which is the insertion of the STMs as I mentioned earlier in this thread to break up any potential onset of feedback. This can be heard by some folks and may be interpreted as fluttering as well. But this fluttering is considered the lesser evil between itself and the feedback, and it’s not always heard by everyone all the times, so Oticon deems it an acceptable side effect/trade-off for feedback prevention.