1. What version of Siemens software are you using (XA30, XA60, XA61)?
2. Obviously, releases of dcm2niix that predate XA60 will not support the private tags from that version. You will want to use the latest stable version (v1.0.20241211):
  https://github.com/rordenlab/dcm2niix/re...
3. You can confirm that the latest stable version of dcm2niix extracts slice times from both Siemens Product as well as CMRR research sequences by converting the validation dataset:
  https://github.com/neurolabusc/dcm_qa_xa...
4. If this still fails, upload one of the JSON files created by dcm2niix when it converts an image.