That is not expected behavior and is not documented as such.
I've tested this on the latest release - Nasm v2.09
using the following and indeed get a reversed listing
nasm -f win32 toktest.asm
%define foo 'every rose has its thorn'
%deftok tok foo
%warning Foo = foo, Tok = tok
and get:
toktest.asm:4: warning: Foo = 'every rose has its thorn', Tok = thorn its has rose every
I would think a bug report is required.