This is just exactly the way jerk works. Segments you were printing had sufficient angle between them to cause the instantaneous speed change to take effect, and the print head would slow down to (some proportion) of the "jerk" speed between the segments. Your high acceleration allowed for a lot of speed to be reached in a small distance, hence the odd seeming behaviour.
When you increased the jerk speed, it no longer had to slow down so much for those segments. You will notice, though, that sharp corners have lots of ringing now.
Decreasing the acceleration would have had a similar effect, but backwards: the print head would not be able to speed up as much between the vertices, so the negative effects would have been slightly mitigated -- though, the print speeds would be much slower.
Don't worry about it, though. Very smart people are at work planning an improved motion system for RRF.
