Wait!
If I do
$ echo test| mail -s xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx $USER
indeed, like you said, the subject is wrapped and I get to see all the
x's. Good.

Same if I use -s "..xxx xxx..." (putting some blanks in the subject.)

But, if I pipe the attached message into procmail,
then in gnus I see the subject truncated there in the Article buffer!