I've seen this problem, and I can partially reproduce the one you
had (but not with total overstrike, just missing spaces). The
problem seems to be where the layout engine knows it needs to
start a new run in the middle of a line. The length of the previous
run on that line is too short, so it starts a new run prematurely.
I'm not sure what's causing this error.
The problem would show up if you had typed a document and then
simply printed it. When you save a document to disk, however,
all the runs are merged into larger chunks. Each span of text
with all attributes but character code the same (font,
size, weight, strike, underline are same) gets merged into
a single run.
If you open this document back up, the break is no longer there in
the middle of the line. The problem goes away.
-- Shaw Terwilliger