The sources do not include an accidental over the mordent. In this harmonic context, the use of b
is obvious, hence in the main text we add a 
over the ornament. Chopin hardly ever put accidentals over trills or mordents in such situations, generally leaving the task of finding the right sound of the auxiliary note to the performer's imagination.
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category imprint: Interpretations within context; Editorial revisions
issues: Omissions to cancel alteration, Inaccuracies in A
notation: Ornaments