Inner blocks of a missing parent are lost on save

[brandonpayton]

Describe the bug
When a post contains a missing (i.e., unregistered) parent block containing inner blocks, its inner blocks are omitted from the content when the post is next saved.

To reproduce
Steps to reproduce the behavior:

1. Open the block editor to create a new post.
2. Add a columns block with a paragraph in each column.
3. Safe the post as a draft.
4. Open the same post in the Classic Editor and note the presence of the wp:columns block and its inner blocks.
5. Rename the opening and closing <!-- wp:columns --> delimiters to use the name wp:columns-missing and save the draft.
6. Reload the Classic Editor and note the presence of the wp:columns-missing block and its inner blocks.
7. Open the post in the block editor and note the missing block notice.
8. Add a paragraph block and save again.
9. Open the post in Classic Editor and note the presence of wp:columns-missing block and the absence of inner blocks.

Expected behavior
The inner blocks of the wp:columns-missing block would be preserved in the post content.

[aduth]

Can you explain the difference between this and #13391 ? I see this has been linked to from #15078 which is directly related to the other issue.

[brandonpayton]

At first, I thought it was the same issue. IIUC, the main thing about #13391 is that inner blocks cannot be nicely extracted from missing parent blocks.

[brandonpayton]

I'm not sure that was clear enough. I believe the difference is that #13391 focuses on getting the blocks out of a missing parent so they can be used apart from the parent, while this issue focuses on content disappearing on save.