When Gnus decodes an email message with

Content-Type: text/plain;
 charset=ISO-8859-1;
 DelSp="Yes";
 format="flowed"

it does not remove the extra space inserted before newlines.

Example encoded text (wrapped at column 70, two spaces after "big" and
before the newline):

The moon in June is like a big balloon, the moon in June, a big  
balloon.

Decoded text (using a large value for fill-flowed-display-column, so no
line breaks, still two spaces after "big" and before "balloon"):

The moon in June is like a big balloon, the moon in June, a big  balloon.

Gnus v5.13
GNU Emacs 25.2.1 (amd64-portbld-freebsd11.0, X toolkit)
 of 2017-04-30

^ I also tried this with a recent build from the master branch and got
the same result.