Won't fix
When using the legacy plugin Layouts and assigning Layout templates to posts, in the post edit screen the Layout metabox is missing. If the post is saved any Layout assignment is subsequently lost.
This only happens when using Toolset Blocks.
As Layouts is a legacy product, the expectation is that it will be used with the Toolset Views plugin rather than with Toolset Blocks.
Activate Toolset Views instead of Toolset Blocks and the normal post assignment functionality will be restored.