ComfyUI cascade stage b implementation introduces artifacts

The two images below were generated using the same stage c conditioning latents, and the same sampler/scheduler (euler, simple, 10 steps). The first image was generated using ComfyUI, the 2nd image using the native diffusers stage b implementation. This behaviour appears consistently across different samplers, schedulers and with different stage c conditioning. Using the diffusers implementation of stage a to decode ComfyUI stage b output latents does produce artifacts, suggesting that the issue is specifically in the stage b implementation, not stage a.


Comfy workflow and example latents (had to zip, github doesn't like the format)
[workflow_test.json](https://github.com/user-attachments/files/17450497/workflow_test.json)
[test_latent.zip](https://github.com/user-attachments/files/17450501/test_latent.zip)