\iow_indent:n doesn't work with \iow_term:n when using \iow_wrap:nnnN

[vermiculus]

\input expl3-generic \relax \ExplSyntaxOn

\iow_wrap:nnnN { \iow_indent:n { test } } { } { } \iow_term:n
% Expected: |    test
% Result:   | test
\iow_wrap:nnnN { \prg_replicate:nn {30} { test~ } } { \iow_indent:n { } } { } \iow_term:n
% Expected: | 30 * 'test' with four-space hang
% Result    | garbage

\bye

$ pdftex mwe.tex 
This is pdfTeX, Version 3.14159265-2.6-1.40.15 (TeX Live 2014) (preloaded format=pdftex)
 restricted \write18 enabled.
entering extended mode
(./mwe.tex
(/usr/local/texlive/2014/texmf-dist/tex/latex/l3kernel/expl3-generic.tex
(/usr/local/texlive/2014/texmf-dist/tex/latex/l3kernel/expl3-code.tex)
(/usr/local/texlive/2014/texmf-dist/tex/latex/l3kernel/l3pdfmode.def)
(/usr/local/texlive/2014/texmf-dist/tex/latex/l3kernel/l3unicode-data.def))
 test
test test test test test test test test
    ^^O^^W^^_^^W^^R^^A^^P indent    ^^O^^W^^_^^W^^R^^A^^P unindent test test test 
test test test test test
    ^^O^^W^^_^^W^^R^^A^^P indent    ^^O^^W^^_^^W^^R^^A^^P unindent test test test 
test test test test test
    ^^O^^W^^_^^W^^R^^A^^P indent    ^^O^^W^^_^^W^^R^^A^^P unindent test test test 
test test test
 )
No pages of output.
Transcript written on mwe.log.

[blefloch]

The first use (putting the text to indent as an argument to \iow_indent:n) is the correct one, and if you do

\iow_wrap:nnnN { \iow_indent:n { \prg_replicate:nn { 30 } { test~ } } } { } { } \iow_term:n

you'll see that the first line gets only one space while further lines get more. I'll try to find a fix.