If the installation repositories are located on the local disk then it is mounted by libzypp before storage probing. And that confuses the storage library.
It is safe to unmount the repositories, libzypp will mount them again when needed
Solution
Release (unmount) all installation repositories before probing storage
Coverage increased (+0.0002%) to 97.777% when pulling b67ef8cd8e24f5537e169233fa328c61ab86e1f3 on pre-SLE-15-SP4 into d4446857d95db1cf74068a8e50fddf6367d988e2 on master.
Problem
Solution