bus assassination mission GTA V

This is intended behavior.

You kill the CEO of a company, the company goes crazy and the stock starts going down over time until they get back on their feet and the stock prices start going up again.

This is actually said by Lester when you start the mission:

Lester: Penny's one of those tight-fisted billionaires, rides the same bus to and from work every day.

Lester: I was thinking you could take over the route and...

Franklin: drop the asshole off where he needs to go. I got it.

Lester: Good. One thing, if I was you, I wouldn't buy any Vapid stock until the acquisition falls through.