I have tried them. Probably not all the tests you did. I think the child locks were off.
I locked the doors from the drivers armrest. The rear doors do not open from the inside, or the outside.
The front passenger door still opens from the inside. The rear electric windows can still be opened by the rear passenger but this doesnt help as the outside door handles still dont work. And if the driver has pressed the electric window override button on the arm rest, the rear passengers cant even open their window.
I tried it in park and in Drive. Same result.
If I remember correctly opening the drivers door cancels the central locking and rear doors can then be opened.
So this does seem to be normal,not a fault with your car. But I agree it could potentially be hazardous. Whilst you dont want passengers opening doors or getting out into traffic, the arrangement virtually allows the driver to keep rear passengers captive. Which is not good if the driver has evil intent , or in the event of a crash where the driver is disabled in some way, or is taken ill.