Right now, if you spend all your resources and buy out a competitor, your stock price takes a massive hit in value, because at that point your stock price was largely inflated based on the stock of competitors owned. After buying the last bit of stock, you have no cash and no stock owned, and therefore nothing boosting your stock price.

I've had several games where someone is getting bought out instantly after completing a buyout due to this stock price tendency

Is this intended or a stock price behavior that's desirable, especially since it doesn't take long after the purchase for the stock to recover?