Qwen3-30B-A3B is slightly slower after mbridge change during refit

repro: `uv run tests/test_suites/llm/grpo-qwen3-30ba3b-8n8g-megatron.sh`

<img width="805" height="709" alt="Image" src="https://github.com/user-attachments/assets/b8c030fe-e4cc-445b-8054-4c22bcaba222" />