This is the complete sequence of 12 images shot at 8FPS (Ch) mode.
AF mode is Dynamic Area AF(Switch Position 2)
Custom A4 Disable Focus Tracking Lock on: On (No)
AF-C, release priority
The AF-On button is configured as the only button to initiate focus and it was held down throughout the sequence.

Image #1 (DSCN_58652): Sensor on black horse. Focus is on black horse
Image #2 (DSCN_58653): Sensor on black horse. Focus is on black horse
Image #3 (DSCN_58654): Sensor on black horse. Focus is on black horse
Image #4 (DSCN_58655): Sensor mostly off black horse. Focus is on black horse
Image #5 (DSCN_58656): Sensor now mostly on crowd. Focus is on black horse
Image #6 (DSCN_58657): Sensor now fully on crowd. Focus is moving back; now about on white horse
Image #7 (DSCN_58658): Sensor still on crowd. Focus is now on the crowd or almost there

The subsequent images just show the end game, but focus is now blown. On image DSCN_58659, no active sensor is indicated in the EXIF.

This series elicits two questions:

1) Why didn't dynamic AF pick up the black horse as he moved into the central sensor?

2) Even if I weren't using a dynamic mode or Dynamic AF failed to do it's job, why didn't Lock-on keep focus for more than two frames?