Vertically stacked text with angle follow

I've seen a few labels happening like in the image below, as reproduced in the linked mapfile.  When ANGLE AUTO is set, the text is rotated as expected; when ANGLE FOLLOW is used, the characters are vertically stacked.  I believe what is happening is that the characters are not getting rotated when the line is vertical.  Making the line slightly non-vertical seems to result in the correct character rotation.

mapfile: https://www.dropbox.com/s/f4q32lg7b6voa6u/labels.map

image: https://www.dropbox.com/s/i3ogv7xvifxc4x0/labels.png