Both dynamic indications in these bars were added by Chopin during a proofreading of FE (→GE,EE). The 
mark was placed at the beginning of the 2nd crotchet in bar 23, which is either a mistake (cf. bar 9) or an example of a notation's convention, sometimes used by Chopin, according to which the mark is not placed at the beginning, yet in the middle of its range of validity (cf. the Etude in C major, No. 1, bar 45).
Compare the passage in the sources »
category imprint: Interpretations within context; Differences between sources
issues: Authentic corrections of FE, Centrally placed marks
notation: Verbal indications