In: https://techbase.kde.org/Development/Tools/Using_kconf_update#Debugging_and_testing the last paragraph. kconf_update doesn't write to update.log per default. It's logs can also be found in .xsessions-errors or in journald
Pending some input on how this should actually work https://markmail.org/thread/ofaeqcabguxyohyb I am fairly confident that you simply should look for the output on stderr though.
Should this be moved to kconfig?
I've updated the page with both the current and future behavior as per the earlier mentioned mailing list thread.