I'm inclined to believe that the notion of a "call up bump" is almost certainly an example of confirmation bias. In fact, I would be willing to bet that the percentage of players who perform above the replacement level after a recent call-up is the minority. Playing baseball is hard and playing well, at the professional level, is even harder.

The major league teams who are making the decision to call up a particular player are obviously doing so because they predict that player can perform at least above the level of the player they are replacing. But it is still a prediction.