Variable not initialized by default in library

I have a library that has a variable with the default set to “” (blank string). When I invoke the flow in a test flow in the same library, everything works ok.
However, running the same tests from a project using the library, I get the error below.
Is this normal?

image (13.9 KB) (13.0 KB)
ArgumentTestLibrary.1.0.1.nupkg (10.2 KB)

This was tested with same results in 2019.4.4 and 2019.8.0