[Bug Fix] Ensure prefill_info_table is populated before honoring disagg_prefill_dp_rank (#22990)

Co-authored-by: Byron Hsu <byron+per@periodiclabs.ai>

python/sglang/srt/disaggregation/decode.py

@@ -421,6 +421,7 @@ class DecodePreallocQueue:
 
             # Fast path: cache-only lookup, no network calls
             prefill_dp_rank = self._resolve_prefill_dp_rank(req)
+            logger.debug(f"prefill_dp_rank: {prefill_dp_rank}")
             if prefill_dp_rank is not None:
                 decode_req.kv_receiver.init(prefill_dp_rank)
                 return
@@ -428,13 +429,14 @@ class DecodePreallocQueue:
             self.pending_reqs.append(decode_req)
 
     def _resolve_prefill_dp_rank(self, req: Req) -> Optional[int]:
-        if req.disagg_prefill_dp_rank is not None:
-            return req.disagg_prefill_dp_rank
-
         prefill_info = self.kv_manager.prefill_info_table.get(_bootstrap_addr(req))
+        # If None, it will go to the slow path and resolve prefill_info by _ensure_prefill_info then cache it
         if prefill_info is None:
             return None
 
+        if req.disagg_prefill_dp_rank is not None:
+            return req.disagg_prefill_dp_rank
+
         if prefill_info.dp_size == 1:
             return 0