Having CP taken away after powering up a Pokemon.

Solution 1:

The CP of the Vaporeon you just powered up has increased correctly, the thing here is that you have more than 1 Vaporeon and there is a glitch that when you power up a pokemon and it's CP increases more than your next higher CP, it switches place, so it seems you lost some CP on that one.

Like this:

1. Dragonite CP 1020
2. Vaporeon CP 1010
3. Vaporeon CP 1000

If you power up the #3 CP 1000 to CP 1015, it will switch on the list to #2 but previously you were seeing the #3 on the list, so now it will display the CP 1010 and will seem to lose some CP, but you are looking at a different pokémon.

1. Dragonite CP 1020
2. Vaporeon CP 1015 ← powered up
3. Vaporeon CP 1010 ← seeing this

The same will happen after you power up the CP 1015 again:

1. Vaporeon CP 1030 ← powered up
2. Dragonite CP 1020 ← seeing this
3. Vaporeon CP 1010

You powered the Vaporeon CP 1015 to CP 1030 but that one was the #2 on this list and became #1, but now you see your Dragonite as it becomes the #2 on the list.

I hope Niantic fixes this soon.