Not sure if this is a bug or if this expected behaviour but I think the post_meta should save if the attributes have been registered in PHP (and not in JS).

To Reproduce
Steps to reproduce the behavior:

1. Register a dynamic block with php containing at least one meta attribute (making sure post type supports rest and meta has been registered)
2. Create the required JS with no attributes
3. Create new post enter
4. Save post

Expected behavior
Post meta is saved/updated

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• windows 10
• chrome
• 67

Additional context

• Gutenberg version 3.0.1.

If I include the following in my block JS the meta value gets saved but if this is only included in PHP it does not:
attributes: { myMeta: { type: 'string', source: 'meta', meta: 'my_meta' } },