It's with the F sharp in the melody, and an EB-Bb in the left hand, right?
I'd view it is a passing tone/appogiatura, though the expected resolution doesn't come the, well, expected way.
In bar 18 we hear we are in E flat major - we have a Bb7 on the first two beats, resolving to E flat major on beat 3, and the last eighth note in the bar again implies a Bb major that we expect to again resolve to E flat major.
In the next bar, he uses the F sharp in the right hand and the EB-Bb in the left hand. We'd typically expect him to resolve the F sharp to a G, so we get an E flat major chord; the F sharp is just a passing tone/appogiatura - this is a very typical gesture Mozart often uses to decorate his melodies. He kind of does that during the 2nd beat with the Ab going to a G in the melody. It's just that in the left hand he changes the B flat to a B natural. So we no longer get the feeling that he has solidified the harmony into an E flat major chord, instead it sounds ambiguous, it's not resolved.
He instead uses this B natural to voice lead us chromatically to the chord in beat 3, overall beat 3 makes up a F major 7 chord resolving to the B flat major on beat 1 in the next bar, which in turn resolves into E flat major which in turn moves us to a cadence into B flat major.. So I'd view it as instead of getting our expected resolution in bar 19, he surprises us and keeps the harmony unstable until finally resolving it in bar 20.
Topic: Mozart - sonata k333 in Bb, mov II