I have noticed that if I zone with a phasefront, I lose the Runspeed buff it gives. And if I then proceed to terminate the Phasefront, I lose an additional 800 Run Speed. This does not clear until I've zoned. Sounds like something to investigate for 18.7.2
Edit:
Happens for me on both:
Phasefront Besom - Sky Sweeper
Phasefront Spectre - Black