The issue got fixed on RHEL9 (sudo-1.9.5p2-10.el9_3).

When executing a ksh script under sudo and sudo has a line to verify the checksum, then the target's user shell is used instead of ksh to interpret the script, as shown in the example below:

ksh script /usr/local/bin/script.ksh

#!/usr/bin/ksh
echo "SHELL is $(readlink /proc/$$/exe)"

configure sudo to verify the checksum

# echo "kshuser ALL=NOPASSWD: sha256:$(sha256sum /usr/local/bin/script.ksh)" > /etc/sudoers.d/kshuser

execute the script under sudo

(kshuser) $ sudo /usr/local/bin/script.ksh
SHELL is /usr/bin/bash

Please backport this from RHEL9, sudo+checksum is an important feature.