How is `CMAKE_NINJA_OUTPUT_PATH_PREFIX` supposed to work?

Attempting to integrate two CMake-generated Ninja projects that use CMAKE_NINJA_OUTPUT_PATH_PREFIX with subninja directives fails. See the following interaction:

alex@alex-ubuntu:~/Development/subninja-demo$ tree
.
├── A
│   └── CMakeLists.txt
├── B
│   └── CMakeLists.txt
└── build.sh

2 directories, 3 files
alex@alex-ubuntu:~/Development/subninja-demo$ cat A/CMakeLists.txt 
cmake_minimum_required(VERSION 3.21)
project(A LANGUAGES NONE)
alex@alex-ubuntu:~/Development/subninja-demo$ cat B/CMakeLists.txt 
cmake_minimum_required(VERSION 3.21)
project(B LANGUAGES NONE)
alex@alex-ubuntu:~/Development/subninja-demo$ cat build.sh 
#!/bin/bash

cmake -G Ninja -S A -B build/A -DCMAKE_BUILD_TYPE=Release -DCMAKE_NINJA_OUTPUT_PATH_PREFIX=A
cmake -G Ninja -S B -B build/B -DCMAKE_BUILD_TYPE=Release -DCMAKE_NINJA_OUTPUT_PATH_PREFIX=B

cat >build/build.ninja <<HERE
subninja A/build.ninja
subninja B/build.ninja
HERE

(cd build && ninja -v)

$ ./build.sh 
-- Configuring done
-- Generating done
-- Build files have been written to: /home/alex/Development/subninja-demo/build/A
-- Configuring done
-- Generating done
-- Build files have been written to: /home/alex/Development/subninja-demo/build/B
ninja: error: B/build.ninja:94: multiple rules generate /usr/share/cmake-3.21/Modules/CMakeDetermineSystem.cmake [-w dupbuild=err]

alex@alex-ubuntu:~/Development/subninja-demo$ cmake --version
cmake version 3.21.3

CMake suite maintained and supported by Kitware (kitware.com/cmake).