I've observed the same effect with a BenQ DW1620.
Here's a burn where the drive decided to slow down from 12x P-CAV to 8x CLV around 2.7 GB, resulting in higher jitter and higher PI error rate at lower RPMs.
I have no idea what triggered the strategy switch. There are no apparent clues in the chart. Other discs from the same spindle were written at 12x P-CAV and 16x CAV to the end with no slowdown.