The easiest explanation for the missing 
hairpin is an oversight by the copyist in [FC] or by the engraver of FE. The possibility of Chopin having removed this mark in one of these sources is less likely, according to us, although it cannot be ruled out, e.g. to suggest a gradual diminuendo from bar 37 to 
in bar 41. The latter was most probably added by Chopin, hence the removal of could be an element of a more comprehensive change to the dynamics of the final fragment of the introduction.
Compare the passage in the sources »
category imprint: Differences between sources
issues: Errors in FE, Authentic corrections of FE
notation: Articulation, Accents, Hairpins