Analysis of information sources in references of the Wikipedia article "X-Ray Imaging and Spectroscopy Mission" in English language version.
...Resolve's spectra are still limited to 1,800 eV and above because the Dewar aperture door to protect the sensitive detector has yet to be opened....
The door, designed to protect the detector before launch, has not opened as planned after several attempts. The door blocks lower-energy X-rays, effectively cutting the mission off at 1,700 electron volts compared to the planned 300.