CMake not including library files when I use FetchContent - c++

Question

I'm working on a project where the CMake structure looks like this:
CMakeLists.txt
cmake_minimum_required(VERSION 3.14)
set(CMAKE_CXX_STANDARD 20)
project(overflow VERSION 0.1.0)
add_subdirectory(src)
enable_testing()
include(CTest)
add_subdirectory(test)
set(CPACK_PROJECT_NAME ${PROJECT_NAME})
set(CPACK_PROJECT_VERSION ${PROJECT_VERSION})
include(CPack)
src/CMakeLists.txt
find_package(Boost REQUIRED)
set(HEADER_FILES overflow.h)
include_directories(${Boost_INCLUDE_DIRS})
add_library(overflow ${HEADER_FILES})
link_libraries(overflow PRIVATE Boost::uuid) # Fails with `target_`
These work fine. However, I get an error when I set up GTest as shown below in my test directory.
test/CMakeLists.txt
## Setup GTest
include(FetchContent)
FetchContent_Declare(
googletest
URL https://github.com/google/googletest/archive/e2239ee6043f73722e7aa812a459f54a28552929.zip
)
### For Windows: Prevent overriding the parent project's compiler/linker settings
set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
FetchContent_MakeAvailable(googletest)
add_executable(overflowtest overflow.i.t.cpp)
target_link_libraries(overflowtest PRIVATE
overflow
gtest
gtest_main)
include(GoogleTest)
gtest_discover_tests(overflowtest)
CMake builds correctly, but when I run the test, the error in overflow.h is:
boost/uuid/uuid.hpp: No such file or directory
What I find strange is that if I change my settings so that test/CMakeLists.txt looks like:
test/CMakeLists.txt
find_package(GTest CONFIG REQUIRED)
add_executable(overflowtest overflow.i.t.cpp)
target_link_libraries(overflowtest PRIVATE
overflow
GTest::gtest
GTest::gtest_main)
There are no errors and it runs correctly. Can someone please explain why using the FetchContent method recommended by Google seems to break my project?