The manual operation test is helpful, but it is not conclusive. The magazine spring has to be strong enough to push the bolt stop up and fully into place in the (very) short window of time that bolt is far enough back to allow it.
So, it could pass the manual operation test, but still fail a live fire test.
Last round bolt hold open can also depend on the ammunition, since it is the ammunition that will determine how quickly the bolt is moving. Too slow, and it won't move the bolt far enough backwards before the recoil spring stops it and sends it forward. Too fast, and the bolt can bounce forward after it has hit the end of its travel, again reducing the amount of time that the bolt stop has to work. Straight blowback firearms are particularly sensitive to a balance between the springs, reciprocating mass, and ammunition.