Change screencapture format on 10.14/10.15

Has anyone had any success changing the screencapture format to jpg on 10.14/10.15 machines? It will sometimes work when changed locally on a machine, but we are planning to roll it out to several machines with a policy/script and it does not seem to work so far, this is what we've been using:

#!/bin/bash

LOCAL_USERS=$(/usr/bin/dscl . -list /Users | /usr/bin/grep -v "^_")

 for USERNAME in $LOCAL_USERS; do
        #Changing screenshot format to jpeg
    /usr/bin/defaults write 
       /Users/$USERNAME/Library/Preferences/com.apple.screencapture type jpg

done

exit 0

Does anyone know why this is not working?

Thanks in advance!