Yeah, that file (and a lot of other config files) is actually generated from template files every time certain variables (in this case: the network configuration variables stored in the Univention Config Registry) are changed. Therefore manual changes get lost.
If you want to know more about templates in Univention and how to integrate manual changes safely (and finding out if manual changes are needed in the first place or if there’s actually a supported way of achieving what you want), you can read a blog post I’ve written about it a couple of weeks ago.