The overrideAcqObsWavelength should be used for migration only. There is a long note and a task reference in InstGNIRS:
// REL-2646. This is an unfortunate requirement that falls out of REL-2646.
// The observing wavelength for acquisition observations should be computed
// based on the imaging filter. Unfortunately in the past this was not
// done and instead the observing wavelength was always taken from the
// grating central wavelength during sequence construction. Old
// observations therefore used the wrong observing wavelength and we must
// continue to compute the value that was actually used for them. To avoid
// setting the observing wavelength during sequence construction for old
// observed observations, we have to track a property in the model that is
// set during migration.
//
// If "override acquisition observing wavelength" is true (which it will be
// by default for new observations), then the new method of setting the
// observing wavelength from the filter will be used. If not, the old
// method of using the grating central wavelength will be used.
Reading through the REL-2646 I'm as confused as ever but the point is that this field will be read in the OCS2 import code and used to set other values but not actually stored in the step. We can work out how this value impacts the settings in another task (which we should add as an issue).
In #249 @swalker2m explains: