Steps to reproduce:

1. Create a new post and add a table block.
2. Accept the default column and row counts and click "Create."
3. Add some text to each cell in the column.
4. Save changes.
5. Refresh the page.

Result: text in some or all of the table cells does not get saved. (30s)

^{Seen at http://alittletestblog.com/wp-admin/post.php?post=15433&action=edit running WordPress 4.9.8 and Gutenberg 4.2.0-rc.1 using Firefox 63.0 on macOS 10.13.6.}

/hat tip Mario Neuhold for the original bug report at https://wordpress.slack.com/archives/C02QB2JS7/p1541020677377700