Inconsistent behavior of Cmake on a library imported by fetchcontent: Inability to find the header file

Question

I have the following directory structure for my cmake project (it is a minimally working example for the sake of this post). I am using nlohmann_json library for my project. I can create jsonExample1 and can successfully run jsonExample1.exe but having difficulty building jsonExample2 as Cmake complains about not being able to find #include <nlohmann/json.hpp>. Both jsonExample1 and jsonExample2 have the same file content for their source files.

Could someone kindly let me know what I am missing that is preventing jsonExample2 from being built? I want to be able to build jsonExample2 from the top level CMakeLists.txt (is this not recommended? In my actual project, I will be builing a library that uses nlohmann for the main executable of the project).

myapp
│
│ CMakeLists.txt
│
├───example
│ CMakeLists.txt
│ json.example.cpp
│
├───external
│   └───nlohmann_json
│           CMakeLists.txt
│
└───src
  json.example.cpp
  

The content of the top-level CMakeLists.txt is as follows:

cmake_minimum_required(VERSION 3.24)
project(my-app VERSION 1.0.0)
set(CMAKE_CXX_STANDARD 20)
set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
add_subdirectory(external/nlohmann_json)
add_subdirectory(example) # it contains jsonExample1
add_executable(jsonExample2)
set_target_properties(jsonExample2 PROPERTIES LINKER_LANGUAGE CXX)
target_sources(jsonExample2 PRIVATE src/json.example.cpp)
target_link_directories(jsonExample2 PRIVATE nlohmann_json::nlohmann_json)

The content of the top-level /external/nlohmann_json/CMakeLists.txt is as follows:

cmake_minimum_required(VERSION 3.24)
project(libjson VERSION 1.0.0)
set(CMAKE_CXX_STANDARD 20)
include(FetchContent)
FetchContent_Declare(nlohmann_json GIT_REPOSITORY https://github.com/nlohmann/json.git)
FetchContent_MakeAvailable(nlohmann_json)

The content of the top-level /example/CMakeLists.txt is as follows:

cmake_minimum_required(VERSION 3.24)
project(projExamples VERSION 1.0.0)
set(CMAKE_CXX_STANDARD 20)
add_executable(jsonExample1 json.example.cpp)
target_link_libraries(jsonExample1 PRIVATE nlohmann_json::nlohmann_json)

The content of src/json.example.cpp and example/jsonExample1.cpp are the same and is as follows.

#include <iostream>
#include <nlohmann/json.hpp>
using json = nlohmann::json;
int main() {
  json j = "{ \"happy\": true, \"pi\": 3.141 }"_json;
  std::string s = j.dump();
  std::cout << s;
  return 0;
}