Glad you got to the bottom of it. Indeed, the cutoff is going to be critical here. The default is set assuming your fODF has been estimated using (MSMT-)CSD with an appropriately calibrated response function, and/or intensity normalised using mtnormalise. This being simulated data, there’s a good chance the scaling differs from what’s expected, which would require a different cutoff.
Otherwise, if you used regular CSD (not the MSMT variant), the default cutoff is probably now too low for noisy data, especially if unconstrained using ACT – similar issue to this thread.