Hello NGE,
this is my first time going through the process of Sponsorship Outland, so I'm not an authority. I'm more of a support network of people like you in the process.
I'd imagine, owing to your wife's pregnancy, that the omission of an x-ray is to be expected and may not be required, subject to other diagnostic tests the Panel Doctor performed. Were you able to ask them about this yourself? Maybe you weren't present, did your wife ask?
You could contact the Doctor with this question for their advice on the matter.
To my knowledge, the x-ray is required to test for Tuberculosis, though, there may be other tests (sensory, chemical) that can be performed to determine the presence of TB or not. If your wife comes from a country where TB is not common, or deemed to be unlikely, it may be no concern at all in the medical.
I wish you and your wife well, and send you my warmest regards for the health and happiness of you all during the pregnancy and birth.