NOTATIO

The following scale passage appears in Mozart’s manuscript copy of last movement of the piano sonata K. 333: The first edition attempted to follow Mozart’s notation but had to split the scale passage and slur between the two systems: The questions facing an editor are

A. Why did Mozart beam the scale passage in two parts? Was this simply a lack of space?
B. Why is only the first part slurred?
C. Why do the slur and beam break at the passing tone C instead of a chord tone?

I believe that the notation is significant because it brings out an inner melody within the passage, as seen in the following diagram: Interpretation

!. The passage outlines the melodic progression: F-B flat-D-F within a 6/4 chord.

2. The slur outlines the passing motion between the F and B flat and links the B flat to the following passing note C to create a two-note motive B flat-C.

3. The beam breaks to show the significance of the D within the melodic progression which is lead to the passing note E flat, creating the motive D-E flat, a transposed repetition of the two-note motive B flat-C, and then to the final goal note F, which is the pitch the passage started on, three octaves below.

4. The final part of the passage is un-slurred, possibly to indicate a more brilliant, detached playing of the ending.

{Edit} The following diagram makes the voice leading clearer. Note that the D in measure 3 could be viewed as an essential tone in the overall melody, which would explain Mozart's desire to highlight the note by breaking the beam before it:
However, after the first edition, many editors have extended the slur and beam by one note and made other changes to the passage: Extending the slur to the D simplifies the melody to place much greater emphasis on the passing note E flat: Slurring and beaming the whole passage as one unit does away with all melodic or motivic play: I leave it to the reader to decide between the interpretations.

Here is another illustration Mozart's subtle slurring; this time from the first movement of the piano sonata K. 333:

Interpretation

While some editors have made the slurs for A-C conform because the measures have the same rhythm, closer inspection shows that the notes of intense measure A may be intended hold through each other to form a four note diminished seventh under the exclusively chord tones in the melody. This extensive over-holding is first established as a motive at the beginning of the movement and is a special characteristic of the piece:
So the left hand of measure one would be played:
But the slurred groups in B and C are best only held through slightly so the final quarter note is left on its own. This creates a lighter, less intense effect that befits the changed emotional state of these two measures. That this is not an error or imprecision is confirmed by the identical slurring of B and C in the first part of the movement. The quick change of feeling from measure A through C is, of course, a characteristic of Mozart's style. Measure A is passionate and almost tragic, measure B relaxes the tension, and measure C is carefree.

Regarding the first post. My take on why the new beam starts at d5 is that the chord in this cadenza is still on the 6/4 (Bb/F). That is, I believe, the reason.

Thanks harpsi. Then it appears that we are in agreement that Mozart's notation is exactly what he intended and should not be second-guessed and modified.

In modifying the passage perhaps later editors have been influenced by Beethoven passage work of the following type in which sub-scales are created within larger ones:
Here is another passage from the second movement of Mozart's piano sonata K. 457 in which he starts a new beamed group on the principal chord tone.
If one looks closely one sees how he writes the A flat at the arrow more carefully than the notes before it, which were sketched in quickly so the stems had to be added afterward and rather sloppily.

Thank you for this fascinating read, John!
Since you clearly sound like having a plan , how will you engrave it?!

Thank you very much, Neera. I am glad you found it interesting.

Yes, I engraved it exactly as Mozart wrote it!

I had no doubt! :-D