Closing as this is currently an expected limitation that will be automatically resolved with the official adoption of RenderGraph in future versions of Unity.
Currently (with the non-RenderGraph path) the depth-stencil buffer is discarded after post-processing and can no longer be bound as an attachment.
In future this should instead work since RenderGraph will be able to preserve the depth buffer beyond its current lifetime (after post) on behalf of the RenderObjects pass declaring it as an RT attachment there.