Chansey buddy shows -0.9/5 km

since you get a candy for every 1.25k walked for the event, you got 3 candies for the km you already walked. I had a negative too. I changed my buddy then changed back and it went to 0.

I changed my buddy and restarted the game. Seemed to work.

try restarting the game before you change buddies and loose a KM. It usually fixes it (during non event times)

Yeah i restarted and it fixed it for me without having to change buddies. Although it only saved 0.1 km

That caught me offguard when the event started but it makes sense.

My Eevee I was walking to turn into an Umbreon might have gotten messed up because of this. He was over 8 KM walked, then the event started and suddenly his total distance dropped to .8 KM, and -1.7/5 for candy. I figured I'd just worry about him later and swapped him out, but now I can't log back in to see what would happen if I made him my buddy again.

While walking my larvitar as my buddy this evening I also saw the negative numbers. It was very puzzling, but I still seemed to receive candies about on cue with distance walked. I read these posts and, with a -0.5/5 km showing on my buddy's screen, I powered my phone off and on and reopened Pokemon Go. Now it shows 0.8/1.25. I believe the negative number indicated how far I had left to walk to get the next candy award. Now it shows how far I have walked toward toward the next candy award. My conclusion - a harmless glitch corrected by powering off and back on.