That dialog works fine in Lubuntu default theme because it has GTK3 support. Those engines you mention are not GTK3, they are GTK2, so the Update Manager (compiled in GTK3) doesn't work fine with them.
That dialog works fine in Lubuntu default theme because it has GTK3 support. Those engines you mention are not GTK3, they are GTK2, so the Update Manager (compiled in GTK3) doesn't work fine with them.