This is the wanted comportment for google apps script library:
That's designed to make private functions.
as it say in the documentation:
If you want one or more methods of your script to not be visible (nor
usable) to your library users, you can end the name of the method with
an underscore. For example, myPrivateMethod_()
As "library function",
function buildForm() can call
function getLabelText_() that is also in that library.
But you can't call directly
function getLabelText_() from the
script using the library. Neither your script has the right to call
So when used as server handler the function
called from the script not from the library, and it won't work. To call
this function you should remove the "_" at the end of it and call it this