I tried to debug it. The function drupal_get_form() has never been called by ...

I tried to debug it. The function drupal_get_form() has never been called by the page "/node/49760/delete"

This is not something that should happen, are you sure you have not overridden the Menu item node/%node/delete and its callback via some other module? Because if it is via drupal_get_form, then the print should show up. The other explanation is that the flow breaks before it reaches drupal_get_form, you can try Xdebug and see where the flow is breaking. Also check if this is happening for all content types, maybe that can give you some clues.