This question is underspecified to give a good answer. Can you include a copy of the JSON file that will reveal the dcm2niix version used (e.g. v1.0.20201102), the manufacturer (e.g. Siemens) and the model (e.g. Vida XA10).

Based on incomplete information, one possibility is that this data Siemens XA data saved as classic (2D) format. If this is the case, I would suggest exporting the data from the MRI console in enhanced (4D) format to preserve meta-information. 

Regardless, I would be wary of using any tool that attempts to interpret the slice timing tags from the JSON file.