Just joining in with the discussion as I have been doing some custom Block Grid Views and as I am now starting to pick up more complex ones where one of the properties of the block is a MNTP which only stores a GUID/key.
I am unsure how best to resolve this problem and was curious if anyone else who is actively working with custom block previews may have encountered this problem or have thoughts,
To avoid flooding this thread - I did create a new forum post to talk about the specific use case, as I feel this thread is a good conversation between the community and the HQ core CMS team about the complexities of writing custom block previews.