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
$PWDfor a file specified as an argument tosource/.when the file was not found in$PATH. (…)iii. Fixed a bug that caused the shell to search
$PATHfor an argument tosource/.even when it contained a/.