This bug only seems to be related to WordPress 5.4 (beta) as it can't be reproduced on WordPress 5.3 (stable).

Steps to reproduce the behavior:

1. Create a simple meta block based on the following tutorial: https://developer.wordpress.org/block-editor/tutorials/metabox/meta-block-1-intro/
2. Add some text to the TextControl and save the post
3. Reload the page and notice the empty TextControl
4. The post meta key/value can't be found in the database

Desktop:

• OS: macOS
• Browser: Chrome/Firefox
• Version: latest stable versions
• Theme: TwentyTwenty

Additional context:

Working 😀

• WordPress 5.3.2
• WordPress 5.3.2 + Gutenberg 7.6

Not working 🙁

• WordPress 5.4 beta3
• WordPress 5.4 beta3 + Gutenberg 7.6