stat.py automatically generates stat code based on the contents of stat_data.py.
In fast mode the script exits early if the following files have been modified

if not [f for f in filter_if_fast([
            "../dist/dist.py",
            "../src/include/stat.h",
            "../src/include/wiredtiger.in",
            "../src/support/stat.c",
        ], prefix="../")]:
    sys.exit(0)

but we're missing stat_data.py.

This means checking out a fresh branch, removing a stat in stat_data.py, and running s_fast will not trigger the necessary changes to our generated code.