Should OSGi config files and the values in the System Settings application always be in sync?

Issue

I have a module where I do not want to store sensitive data (eg. password) in the config file
I would like to provide this value over the UI in System Settings
Is it possible to use a "mixed" approach?

It should not be possible to use the mixed approach (when you provide some configuration values in the config files, while others are provided in System Settings).
Liferay will keep the OSGi config file and the values on the UI in sync. When the configuration is modified on the UI, the config file is updated with the new value. If the config file is modified, then the values on the UI are updated.
We advise to use the single source approach: either update the configuration on the UI or use the config files.
"Mixed" configuration should not be used. Due to unforeseen issues, it might happen that the settings come out of sync. In such cases, you can easily lose previous settings.