Wavelengths in 1-D extracted spectra and 2-D rectified images will not be correct unless the offset of the Mode Select Mechanism (MSM) in the dispersion direction was successfully determined from the wavecal associated with the observation. The MSM offset is typically a few pixels, which is small enough that it is possible that it could go unnoticed. Here are two things which can be checked to see whether there was a problem.
(1) The CALSTIS-4 section of the trailer file may contain warning messages such as:
Warning Peak in cross correlation is at end of range. Warning Shift in dispersion direction could not be determined.
Reading the trailer file is less fun than reading the dictionary, but you can search on "Warning".
(2) The MSM shift in pixels in the dispersion direction is recorded in the SCI extension headers of the crj.fits and flt.fits files with the keyword SHIFTA1. SHIFTA1 will be zero if calstis failed to find the shift in the dispersion direction, or if wavecal processing was not done at all. While zero is a possible real value, it is of course unlikely that it would be exactly zero.