"Lord, 'Tis Not That I Did Choose Thee"

A little over a month ago, I watched the Concordia University Wisconsin chapel service from 25 January.  The hymn was "Lord, 'Tis Not That I Did Choose Thee" (LSB #573), and I noticed some significant accidentals in the alto part.  Here are the fifth and sixth phrases from the tune "O du Liebe meiner Liebe":

In the first verse, the text here is "Thou hast from the sin that stained me / Washed and cleansed and set me free."  Under the first "me," there's a C#, and the accidental provides some sense of being "stained" by sin, but in the following phrase, it reverts to a C natural under "cleansed," giving some sense of that purity.