Bug? - ISPConfig removes main.cf customisations on UI config

Not sure if feature or bug, but pretty annoying:

1. create customised postfix main.cf lines in "/usr/local/ispconfig/server/conf-custom/install/postfix_main.cf.master"
2. run "ispconfig_update.sh --force"
3. verify your lines are reflected in "cat /etc/postfix/main.cf | grep 'xxx'"
4. open ispconfig3 web ui and go to System / System Config
5. switch the view to "Web" tab
6. it triggers UI config change (red number upper right)
7. the customised main.cf is gone and replaced with ispconfig3 default main.cf

resulting in need to do "ispconfig_update.sh --force" every time you visit System Config and change tabs, even if you do not do any change at all.