test_multiexposure_save.py

import asdf
import jwst.datamodels
import numpy

af = asdf.AsdfFile({'data': numpy.zeros((10, 10), dtype='f4')})
af.write_to('tmp.asdf')
with asdf.open('tmp.asdf', mode='r') as af:
    m = jwst.datamodels.MultiExposureModel()
    data = af['data']
    assert data.__class__.__name__ == 'NDArrayType'
    im = jwst.datamodels.ImageModel()
    im.data = data
    im.err = data.copy()
    im.dq = data.copy()
    m.exposures.append(im)

    m.save('foo.asdf')
    m.save('foo.fits')

Using NDArrayType from asdf produces the same result, a fits file with embedded asdf with no blocks in the asdf.