When I build my docs with mkdocs build --strict, I get an error because of warnings:

Aborted with 5 warnings in strict mode!

But with verbose on (mkdocs build -v --strict), the build finishes:

INFO    -  Documentation built in 10.17 seconds

The output value ($? in a bash terminal) has the same behavior: mkdocs build --strict returns 1 and mkdocs build -v --strict returns 0