When DirectWrite is used and defined font is fixed width font, if a line contains unicode replacement character U+FFFD, DirectWrite will change the font of the whole line to some other proportional font, probably due to some glitch in fallback algorithm. However, this doesn't happen always, if the line consists only of the U+FFFD character, the font will not be changed. With that in mind, changing BreakFinder to add breaks on replacement characters should probably fix the problem.
Attached is a sample screenshot.
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