-s flag (soelim) not working?

[Peter Schaffter]

I recently discovered that documents with .so'd files containing
non-ASCII input require preprocessing with soelim(1).  If we have

file: source
.pl 1v
.ds FOO àéîöù

file: main
.so source
.nop \*[FOO]

running
  groff -Tutf8 -k main
spits out garbage, whereas
  soelim main | groff -Tutf8 -k
correctly renders string FOO.

soelim(1) doesn't mention utf8 input, merely
  "It is useful if files included with .so need to be preprocessed."

Since it is non-evident that utf8 input (as opposed to inputting
named glyphs) in sourced files needs to be preprocessed (i.e.
with soelim), saying something about it in the manpage would be
helpful.  I'm fairly certain most users, on reading "...need to be
preprocessed" think tbl, pic, eqn, grap, chem..., not utf8 input.

Now, the '-s' flag issue...  Unless I am misunderstanding or
misreading something,
  soelim main | groff -Tutf8 -k
and
  groff -Tutf8 -ks main
should produce identical output, however the -s flag is ignored and
groff spits out garbage.  What's up?

-- 
Peter Schaffter
https://www.schaffter.ca