/usr/lib/ardour5/libgtkmm2ext.so is in ardour 1:5.12.0-3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/ardour5/libgtkmm2ext.so.0.8.3
/usr/lib/ardour5/libgtkmm2ext.so.0.8.3