Yes. "Shallow water blackout with a prolonged exhale while breath holding in the pools depths during a moderate workload drill" is real risk.
A neutral article as talking point for further research:
Why holding your breath underwater can be so dangerous | CBC News
The principle is the same with unused scuba gear on our backs. I haven't witnessed it happen during an OW class to a student myself. (I also won't have students perform it that way.)
Here's a little more on this common safety risk:
Aquatic Safety
More Information
Many pools have prohibited underwater breath hold games for this reason and some freediving agencies are limiting breath holding to 60 seconds for basic training (not exhaling while shallow. The risk is considered unexceptable.)
We're not screening for blackout susceptibility in our OW classes and I am concerned it's a matter of time before we have some unconscious students if we make them all horizontal 'CESA' in shallow water.
The recipe is a known one and that is the occasional result.
(I've both witnessed and experienced shallow water blackouts.)