Still no clues on this but got to read heaps whilst looking
. I have sent an email to the mag asking if they can point me to the specific findings.
But this is very curious as the explanations given come down to corrupted data or the galaxies that dont throw a shadow all emit the "same precise 2.725-Kelvin temperature" so as to cancel the shadow. While data could be the problem one could ask why some observations were corrupted but others were not so the mystery would remain. Of course if the particular galaxies emit "same precise 2.725-Kelvin temperature" one wonders why. I dont know the percentage which does not help.
I must say however all the looking showed no reference to the matter of missing shadows but like most things one ends up learning something.
alex