I don’t think this should happen, no: presumably when the resolution is isn’t great in the lf, something is going screwy selecting the bins that contribute to a particular mel filter. That said – not having looked in detail – the algorithm may just not be tractable when the number of bands gets too close to the available resolution. I will enquire of the guru…
The fft size can be greater than numframes, yes, because the fft size is only constrained to be at least the window size. So you could use @fftsettings 256 64 512