Oh dear.. Yes—this is a classic “buy the rumor, sell the news” situation. A successful Starship 13 did not necessarily mean SPCX should rise that afternoon.

The main reasons were likely:

The success was already anticipated and priced in.
SPCX had been under pressure largely because of the previous launch aborts and delays. Once the launch finally succeeded, traders had already bought ahead of the event. The actual success became a reason to take profits rather than initiate new buying. The stock had already been widely viewed as needing a successful flight to stabilize the decline.

The market may have viewed the result as “good, but not transformational.”
The flight accomplished important milestones, including deploying 20 Starlink V3 satellites and demonstrating an engine restart. But it was still a test flight, not proof that Starship is operationally reusable or ready for commercial service. The stock price reflects future cash flows, not simply whether a test flight succeeds.

SPCX had other, larger concerns weighing on it.
Investors were looking ahead to the first quarterly earnings report after the IPO, worried about the company's enormous valuation, future growth, and upcoming potential share-supply/lockup issues. Those concerns can overwhelm one positive event.

Some investors may have expected an even better result.
A key nuance: reports differ in how they characterize the booster outcome. The upper stage achieved major milestones, but the booster did not complete the desired soft landing. If investors had expected full booster recovery, the flight could be interpreted as partial success rather than a perfect success.

Short-term stock price is driven by positioning, not just news quality.
If many traders bought SPCX expecting a successful launch, the launch itself creates the moment to sell. In that sense, “successful launch” and “stock goes down” are perfectly compatible.

My interpretation: the market reaction was not saying Starship 13 was a failure. It was saying that the success was already largely expected, while the market still wants evidence of the much bigger milestones—reliable rapid reusability, meaningful commercial revenue, Starlink growth, and eventual profitability.