Movement speed slowly decaying over instance

I noticed that my character was feeling sluggish, so I watched my movement speed modifier over the course of running some maps. Turns out that my modifier has been slowly decreasing as I kill things... standing around does not cause it to change.

I am running an arc totem Hierophant build.

Reference number 2,654,223,254

Update: did a couple delves, ended up at a spicy -28% movement speed... Makes me think that something is decreasing my speed a tiny bit after every kill.

Your Dream Fragments ring seems the most likely to be at fault. If you trade it out for a rare, does the problem go away?

EDIT: just saw this:
https://www.pathofexile.com/forum/view-thread/2262108
probably the same issue.

Confirmed, the bug is with Soul of Lunaris.