This may or may not prove related to #1912 but when ASSIMP_LIB_INSTALL_DIR contains more than one directory level the logic below isn't sufficient to find the root directory producing a broken CMake configuration.

This affects the libassimp-dev package on Debian Stretch, Buster, and Sid and Ubuntu Artful and Bionic. Verifiable on these platforms with the following example CMakeLists.txt:

cmake_minimum_required(VERSION 3.0)
project(asitest)

find_package(assimp)

message(STATUS "ASSIMP_ROOT_DIR: ${ASSIMP_ROOT_DIR}")
message(STATUS "ASSIMP_LIBRARY_DIRS: ${ASSIMP_LIBRARY_DIRS}")
message(STATUS "ASSIMP_INCLUDE_DIRS: ${ASSIMP_INCLUDE_DIRS}")

Selected output from above CMakeLists.txt:

...
-- ASSIMP_ROOT_DIR: /usr/lib
-- ASSIMP_LIBRARY_DIRS: /usr/lib/lib/x86_64-linux-gnu
-- ASSIMP_INCLUDE_DIRS: /usr/lib/include
...