I went back to see the T98973 task and I saw the second problem that @Jon Denning (gfxcoder) highlighted, I couldn't say if the data really persists when saving the file. Opening a new project and executing the code 'cycles\0anything' in bpy.context.scene in the Blender Python Console I get True as output

Update:
I added a debug print to check the data stored at the name address:

for(int x=0; x<=MAX_IDPROP_NAME; x++)
{
  printf("%x ",self->prop->name[x]);
}

I was able to verify that the data persists when saving and opening the file, but it is no longer accessible by the null terminator.

Using PyUnicode_AsUTF8AndSize, the method gets the full decoded data, even if there is a '\0' in the middle of the input, wouldn't it be better to use PyUnicode_AsUTF8String which truncates the string to the first null terminator?

Additionally I found a new problem renaming properties, we can have 2 or more with the same key
#

import bpy
C = bpy.context
C.scene['1'] = {'foo': 1}
C.scene['2'] = {'foo': 2}

list(C.scene.keys())
#['cycles', '1', '2']

C.scene['1'] = {'foo': 56}         #Override C.scene['1']

C.scene['2'].name = '1'           #Rename successfully

list(C.scene.keys())
#['cycles', '1', '1']

Should I report it as a bug?