Adding a massive +1 to this bug report. We are experiencing the exact same context-injection failure with the postToolUse hooks in v3.1.15.
Just like you described for sessionStart, the postToolUse hook executes correctly and outputs the valid JSON schema for additional_context, but that context is completely discarded and never surfaces to the agent model.
(As a side note, postToolUse is currently also suffering from entirely empty input payloads and intermittent firing, making the whole hooks system feel very fragile right now).
It strongly appears that the core pipeline responsible for taking a hook’s stdout and actually injecting it into the Agent Window is broken across multiple hook types in this build.
For any devs looking into this, here is the related thread detailing the exact same failure point for postToolUse: