I can confirm that I see the same behaviour (100% reproducible when creating new tabs) when using the zukimac theme.
$ sudo dpkg-query --show compiz unity zukimac-gtk-themes compiz 1:0.9.12.1+15.04.20150410.1-0ubuntu1 unity 7.3.2+15.04.20150420-0ubuntu1 zukimac-gtk-themes 1.5
Changing to the standard Ambiance theme and gnome-terminal behaves as expected.
I can confirm that I see the same behaviour (100% reproducible when creating new tabs) when using the zukimac theme.
$ sudo dpkg-query --show compiz unity zukimac-gtk-themes 1+15.04. 20150410. 1-0ubuntu1 04.20150420- 0ubuntu1
compiz 1:0.9.12.
unity 7.3.2+15.
zukimac-gtk-themes 1.5
Changing to the standard Ambiance theme and gnome-terminal behaves as expected.