This is due to the fact that... when the 'Duration Mode' is switched to 'bursts' (as oppose to 'continuous'), the 'Duration' is defaulted to '30' (frames), and the 'Burst Size' is defaulted to '1' (burst interval). When the test is started, it sends only ONE burst, of 30 frames... and stops as it should.
To get the Traffic Generator/test to run for a longer duration, either/or both 'Duration' and 'Burst Size' needs to be increased as needed.