I noticed this pattern in Auden's poetry where the last line in a verse will be much shorter than preceding lines. Here is one of the earliest examples, taken from Paid on Both Sides:
Here a scrum breaks up like a bomb, there troops
Deploy like birds. But proudest into traps
Have fallen. These gears which ran in oil for a week
By week, needing no look, now will not work;
These manors mortgaged twice to pay for love
Go to another. <<<
This is definitely not enjambment. The closest thing that I could find is catalexis, which describes a line of verse lacking one syllable, but this is not quite right since the last line is often shorter by much more than just one syllable.