How to inherit paths from CMakeLists in parent directories?

Question

I have two projects PROJ in my workspace ws with corresponding third party libraries TPL as shown below. All depend on opencv 4.4.

ws   
├── PROJ1 (opencv 4.4)
|    └── TPL1 (opencv 4.4)  
└── PROJ2 (opencv 4.4)
    └── TPL2 (opencv 4.4)

PROJ1, PROJ2 and TPL1 have following lines in their CMakeLists.txt:

set(OpenCV_DIR $ENV{OPENCV4_DIR})
find_package(OpenCV 4.4 REQUIRED) 

Whereas TPL2 only have:

find_package(OpenCV 4.4 REQUIRED) 

but does not have

set(OpenCV_DIR $ENV{OPENCV4_DIR})

When I run cmake and make inside PROJ1/build and PROJ2/build they succeessbully get built. However, notice that PROJ2 gets built without TPL2 having set(OpenCV_DIR $ENV{OPENCV4_DIR}) specified in its CMakeLists.txt, but TPL1 needs to have this line for PROJ1 to build successfully. Otherwise it gives following error:

fatal error: opencv/cv.h: No such file or directory
26 | #include <opencv/cv.h>
   |          ^~~~~~~~~~~~~

Why is it so? Is there some CMakeLists.txt config that lets third party apps to inherit paths from CMakeLists.txt in parent directories or something similar?

PS: I do not do make install in any of these PROJs and TPLs.