awk bug substr

[Gilali]

Hello,
First, thank you very much for all your good work, awk is a tool very 
very usefull.

now, i think a found a real bug :
All awk internal functions are working OK also in utf-8 or in iso-8859-1 
text format. All, except one : susbtr function, see this :


GNU Awk 3.1.8

vi test.awk :

BEGIN{
line="ééé blabla from me" ## ééé character is here for be sure the file 
will be saved in utf-8 format
if (match(line,"from")){
print "matching is OK"
test=substr(line,RSTART,RLENGTH)
print test}}



awk -f test.awk :
matching is OK
from
############ it's normal, but :

iconv -f utf-8 -t iso-8859-1 test.awk -o test.awk
awk -f test.awk:
matching is OK    # but nothing else instead of showing "from"

AND REVERSE

iconv -f iso-8859-1 -t utf-8 test.awk -o test.awk
awk -f test.awk:
matching is OK
from

I think is a bug because all others function working well in iso-8859-1: 
sub, match (tested here), split etc...  only substr not working.

Please tell me if is a real bug

Regards