I'm finding this in vista, so far only with the calibri font. It seems to be triggered by the letter 'f' followed by certain other letters.
//var %x = fl | echo -a test $str(%x,5) test
cuts off the final 'st'. If you change 'fl' into 'flat', it cuts off only the final 't' and part of the 's', probably because a higher portion of the string consists of the offending combinations. Using 'fc' or 'fd' doesn't cause the problem at all.
The chop problem might also be related to the way the strings will sometimes turn some of the letters purple.
The calibri chop doesn't seem to happen if you enable bold, and diminishes as you make the font size larger.