So, this happened to me today. Take a look:
As you can see, I won against the Vega player, and this was my PP at the moment:
After this match, I fought against a Ken player:
Now look at my PP again:
Lost 128 PP because I won… da fuq?
Happened to me too like 4x before, probably some fuck up with GFWL thinking you DCd or something 
has happened to me several times. I started at close to 2500pp won several matches, lost PP on every game. then started to get random disconnects, lost more PP and yeah. In the span of about 10 matches I was nearly under 1kpp. There was nothing wrong with my net, but I did delete my game data and reinstall it. this was also on PSN.
This has happened to me about 3 times, pretty f’ing rage inducing. I wonder if the player i beat gains the points, or if he is effected too.
PSN btw.
Yesterday I lost some 120 PP just for kicking out a player.
Oh ok, good to know that this is a common problem.
Most of the time, I played on PSN, but I also bought the game for PC now and this is the first time I encountered this problem.
This also happened to me yesterday.
Was starting at around 1950 PP and then got a losing streak. After winning one game finally it got worse because I lost pp again, lol. Dropped down to 1050 PP or so.
This happened to me A LOT about 2 months back. I was losing PP from winning my matches so I thought “ok it’s just a momentary glitch.” It then spread out to random disconnects every 2 or 3 matches to the point where I couldn’t even play online anymore. It eventually resolved itself after I reinstalled the game and cleared my cache (I’m on xbox btw).