/usr/lib/x86_64-linux-gnu/libisal.so is in libisal-dev 2.21.0-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libisal.so.2.0.21
/usr/lib/x86_64-linux-gnu/libisal.so.2.0.21