I am not familiar with the codes that are/were in effect in other locations. I can tell you the possibilities that I am familiar with.
1. The ceiling is protected with 5/8" Type-X drywall or equivalent and so are the elements that support the ceiling such as walls and beams.
2. The Ceiling is protected with 5/8" Type-X drywall or equivalent but the elements that support the ceiling are protected by 1/2" drywall or equivalent.
Doors have always been similar to this.....
solid wood doors not less than 1-3/8 inches in thickness, solid or honeycomb-core steel doors not less than 1-3/8 inches thick, or 20-minute fire-rated doors, equipped with self-closing or automatic-closing and self-latching device.
I copied this right out of the CRC "solid or honeycomb-core steel doors not less than 1-3/8 inches thick." I have never encountered an 1-3/8" solid steel door. I must be missing something in the translation.
In my code there's a blurb about the door being "tight fitting" but I can guarantee that my code was never enforced in Tennessee.
There is a third possibility which is what you have currently. If there were no codes adopted or no oversight, then anything is possible.....and perhaps legal. If it were me I would retrofit to possibility #1.