I have been getting some unusual artefactual FSCs when refining a number of different datasets. The FSC reaches a 0.143 threshold at a value which appears consistent with features in the map. There are then some large oscillations in the value after that point, reaching values up to ~0.4 near to nyquist. See attached picture.
Could you suggest what might be happening here and if there are any ways to alleviate the problem?