UNavigationSystem Why MainNavData is null?

Hello,

Am confused when to use MainNavData and AbstractNavData, other than the code comments I haven’t found any documentation.
We are experiencing problems in cooked builds when accessing AbstractNavData.

Switching to MainNavData solves the problem but breaks a different stuff in our navigation .

EDIT: apparently doing a full rebuild and cook fixes our problem. Is it always recommended to enable FullRebuild ? We discuss that for the changes we are making content wise, shouldn’t be necessary