new-exp.tests fails with string manipulation

Running the test 'by hand' it seems to work ok, but running in the harness, it gets truncated.
The particular failing test is in new-exp8.sub:

declare z="start"
declare NUM=1000
pat1='str'
export LINE_MAX=100
#----------------------------------
# create a long string with ';'
#----------------------------------
for ((i=0; i<$NUM; i++)); do
    z="$z;string$i"
done

#z="$z;string;foo"

#------------------------------
# delete everything except ';'
#------------------------------

# try different patterns here
x="${z//$pat1}"
echo $x

The string being echo'ed is getting truncated

Start with the failing test in new-exp8.sub and compare its behavior when run by hand with its behavior in the new-exp.tests harness. Trace how the generated long string is passed to the harness and determine why the echoed value is truncated; done means the test completes without truncating the string.