[BUG] Langfuse incomplete trace when prompt chaining

[AdamSGit]

Describe the bug
When chaining prompts on a flow (see prompt chaining example in flows library), and configure Langfuse to analyse the flow, only the last llm chain, trace and generation is tracked.

To Reproduce
Steps to reproduce the behavior:

1. Create a flow from the prompt chaining example
2. Use a langfuse local instance or the langfuse cloud
3. Configure langfuse tracking in the flow
4. Run the flow

Expected behavior
All the chained generations are correctly tracked by langfuse

Thanks

[HenryHengZJ]

working on it! #2195