É um bug que foi corrigido em uma versão subsequente. Do registro de alterações do bash :
This document details the changes between this version, bash-4.0-alpha, and the previous version, bash-3.2-release. (…)
1. Changes to Bash (…)
ggg. Fixed a bug that caused a shell running in Posix mode to search
$PWD
for a file specified as an argument tosource
/.
when the file was not found in$PATH
. (…)iii. Fixed a bug that caused the shell to search
$PATH
for an argument tosource
/.
even when it contained a/
.