isolatedSession: true does not actually isolate heartbeat sessions

Related: #51542 (locked)

Environment

• OpenClaw 2026.3.28 (f9b1079)
• Linux 6.17.0-1009-aws (x64), Node 24.14.0

Config

{
  "heartbeat": {
    "every": "30m",
    "target": "last",
    "directPolicy": "allow",
    "isolatedSession": true
  }
}

Observed behavior

• A heartbeat session created on 2026-03-27 was reused continuously for 2+ days (543 JSONL entries, same session ID 97ba1b61-*)
• HEARTBEAT.md was updated on 2026-03-29 05:07 UTC (changed "B站UP主" → "微博博主"), but the heartbeat kept outputting the old text from the cached system prompt
• The session contained 111 references to the old text and 0 references to the updated text
• Gateway restart (new PID) did not create a fresh session — the old one was loaded from persistent storage

Expected behavior

Per docs: isolatedSession: true should provide "fresh session each run (no conversation history)". Each heartbeat tick should:

1. Create a new session
2. Re-read HEARTBEAT.md into the system prompt
3. Not carry over context from previous heartbeat runs

Workaround

Manually delete the session file + remove heartbeat keys from sessions.json:

rm ~/.openclaw/agents/<agent>/sessions/<session-id>.jsonl
# + remove heartbeat session entries from sessions.json

Root cause hypothesis

isolatedSession: true is either:

1. Not being checked when deciding whether to reuse an existing heartbeat session, or
2. The isolation logic creates a "new" session but restores from persistent storage using the same session key, negating isolation