Recently I noticed from the constant noise of my Linksys WRT1900AC router that the /sbin/fan_ctrl.sh script which is being called from cron was no longer working.
Some troubleshooting made clear that that is because the echo command in the script requires a "-n" option (do not output trailing newline) to function correctly. At least that change makes the script functional again for me.
This is also on a wrt1900ACv1.
PRETTY_NAME="OpenWrt 21.02.3"
Maybe it is related to the installation of bash?
What is root's shell with you?
If adding ´-n´ would help and doesn't create problems in the default setup it probably could be committed to the production code. I just make the change when updating now. It's not a big problem.