Sounds like you got the error that drops you out of the catch encounter which happens sometimes. It doesn't influence whether the pokemon gets caught or runs, so if the Chansey fled it was just bad luck for you. The error likely prevents a journal marking from being made since your client never sees it run.
GamePress
Disappearing act?
Tried to catch the only bloody Chansey I actually saw during the pink event.
Lobbed a raspberry at it, then an ultra ball. Ball hits, big flash as per usual, ball closes, then FOOF, the catch scene just disappeared.
It's nothing logged in the journal ("Chansey 956 CP ran away") or anything.
At 956 CP it prolly wasn't much to work with, but at least it was something.
Known bug, network error, random fluke?
I had no issues playing the game other than that during the day.
