Resizing xlab with theme_tree2 removes horizontal line

[dstern]

I can add a label to theme_tree2() using xlab, but when I try to resize this label with theme(axis.title.x = element_text(size = N)), the horizontal line on the scale bar is removed. I have tried moving the axis title and many other things. Can't find a solution on Google.
e.g.
This works

ggtree(tree, size=.2, aes(color=group)) + scale_y_reverse() +
theme_tree2() + scale_color_manual(values = c("firebrick","steelblue")) +
xlab(label = "Divergence (amino acid substitutions per site)")

But when I try to alter the font size of the title, the horizontal line disappears

ggtree(tree, size=.2, aes(color=group)) + scale_y_reverse() +
theme_tree2() + scale_color_manual(values = c("firebrick","steelblue")) +
xlab(label = "Divergence (amino acid substitutions per site)") +
theme(axis.title.x = element_text(size = 12))