$LIB variable does not expand to anything in Ubuntu Server 18.10

I have the same exact problem as this question previously posted, where /etc/ld.so.preload does not intercept the right architecture. A little background: I have compiled a shared object (64-bit) that is referenced in the ld.so.preload file on any binary execution. The problem was that I was getting a ERROR: ld.so: object '/usr/local/lib/mysharedobject.so' from /etc/ld.so.preload cannot be preloaded (wrong ELF class: ELFCLASS64): ignored. when executing 32-bit programs. To fix the issue according to the answer in that question, I had to make two directories (lib/i386-linux-gnu and x86_64-linux-gnu, for example, in /var/opt) and specify /var/opt/$LIB/mysharedobject.so in /etc/ld.so.preload so the right library will be preloaded depending in program architecture. So in that case, in Debian based systems, /var/opt/$LIB/mysharedobject.so would expand to: - /var/opt/lib/i386-linux-gnu/mysharedobject.so for 32-bit programs; - /var/opt/x86_64-linux-gnu/mysharedobject.so for 64-bit programs. However, after applying this, any binary I run (such as ls) will output the following 'error': ERROR: ld.so: object '/var/opt/$LIB/mysharedobject.so' from /etc/ld.so.preload cannot be preloaded (cannot open shared object file): ignored. As you can see, $LIB did not expand to anything. I have also set $LD_LIBRARY_PATH to /var/opt and ran ldconfig to update the system with this libs, but with no success. What is the problem here?