Stunning Before Slaughter
This is correct that blood is supposed to drain. But stunning generally has little effect on this. In New Zealand all animals are stunned before slaughtering, and New Zealand is the largest halal meat exporter certified by many Islamic authorities. Slaughtering a conscious animal helps with draining the blood because the heart pumps faster, but it’s not a requirement. As long as the blood is still drained out of a slaughtered animal that was stunned, it’s still considered zabiha.
Apparently there is a large possibility that the animal dies from stunning. Should this be a concern?
There’s only a small possibility of an animal dying from stunning, and if it happens it becomes a matter of ijtihad.