System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce RTX 3070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 460.79

Blender Version
Broken: version: 2.92.0 Alpha, branch: master, commit date: 2021-01-07 22:28, hash: rB61f1faac3f21
Also Broken: version 2.83 LTS (We found this bug today at work)

Short description of error
If the "safe buffers" checkbox (in the render>performance>final render tab) is checked, blender doesn't write/create the metadata for the cryptomatte. This is especially important if blender is used in a compositing pipeline with nuke or fusion.

Exact steps for others to reproduce the error

1. Open a new Blender file
2. Check any cryptomatte option in the Output Properties (I used "Object")
3. Navigate in the Property Panel to Render Properties > Performance >Final Render
4. Check Save Buffers
5. Render the scene
6. In the image editor of the rendered image press "n" or open the side panel
7. Navigate to Image > Metadata

There will be only the metadata information from Output Properties> Metadata
But the cycles metadata and especially the metadata for the cryptomatte are missing.

Here is an example of the metadata:
with Safe Buffers:

whitout Safe Buffers (these metadata are important):