The drop in DoubleVerify's stock price, despite the announcement of a potentially positive development in its partnership with TikTok, suggests that the market may have had higher expectations or there are other factors influencing investor sentiment negatively in the short term. The introduction of new brand safety and suitability categories is a strategic move to enhance the value of DV's offerings to advertisers on TikTok, which could be beneficial in the long term. However, the immediate negative reaction in stock price indicates a short-term bearish outlook by investors.