I am trying to figure out why Notebook tabs lose underscores.

Say I want to have a Notebook with 3 tabs:

 ______  ______  _________________
/ Tab1 \/ Tab2 \/ with_underscore \

(I use all the default settings, these are all Labels behind the scenes if I recall)

What I get with GtkD is the following:

 ______  ______  ________________
/ Tab1 \/ Tab2 \/ withunderscore \

What am I doing wrong?