BUG #16619: Amcheck detects corruption in hstore' btree index (ver 2)

The following bug has been logged on the website:

Bug reference:      16619
Logged by:          Andrew Bille
Email address:      andrewbille@gmail.com
PostgreSQL version: 13beta3
Operating system:   Ubuntu-20.04
Description:

Hello all.
Index corruption detected after upgrade from v 9.6/10:

mkdir v10
cd v10
git clone https://github.com/postgres/postgres.git ./
git checkout REL_10_STABLE
 ./configure >/dev/null && make -j8 >/dev/null && make -j8 contrib
>/dev/null
echo "SELECT 1;" >> contrib/hstore/sql/hstore.sql
make check -C contrib/hstore
cd ..
mkdir v13
cd v13
git clone https://github.com/postgres/postgres.git ./
git checkout REL_13_STABLE
./configure >/dev/null && make -j8 >/dev/null && make -j8 contrib
>/dev/null
echo "EXTRA_INSTALL = contrib/amcheck contrib/pageinspect"
>>contrib/hstore/Makefile
make check -C contrib/hstore
export LD_LIBRARY_PATH=tmp_install/usr/local/pgsql/lib
tmp_install/usr/local/pgsql/bin/initdb -D /tmp/db
tmp_install/usr/local/pgsql/bin/pg_upgrade -d
"../v10/contrib/hstore/tmp_check/data" -D /tmp/db -b
"../v10/tmp_install/usr/local/pgsql/bin" -B
tmp_install/usr/local/pgsql/bin
tmp_install/usr/local/pgsql/bin/pg_ctl -D /tmp/db -l l.log start
tmp_install/usr/local/pgsql/bin/psql -dcontrib_regression -c "create
extension if not exists amcheck;"
tmp_install/usr/local/pgsql/bin/psql -dcontrib_regression -c "SELECT
bt_index_parent_check('hidx', true);"

I get the following error:
ERROR:  mismatch between parent key and child high key in index "hidx"
DETAIL:  Target block=3 child block=1 target page lsn=0/16D5758.

(The same procedure with v11/12 doesn't produce the error.)

Thank you.