I don't think it's the calibre Editor as such. It's something to do with the underlying toolkit calibre uses to build everything. The problem seems to occur when non-breaking-spaces (those yellow spaces you can see) occur at the edge of the window in the wraparound area. If you change the width of the code window I think you will see that the 'missing s' reappears.
[ETA: Too late. Ninja-ed again!]
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