Table Block: table cells in thead are misaligned when "Fixed width table cells" is toggled off

[designsimply]

Steps to reproduce:

1. Create a new table block.
2. Edit it to use the sample HTML below which contains thead and tbody tags.
3. Switch back to "Edit Visually."
4. Toggle the "Fixed width table cells" option in the sidebar.

Result: when "Fixed width table cells" is toggled off, the thead and tbody table cells are misaligned.

Sample HTML:

<table class="wp-block-table"><thead><tr><td><strong>Column One</strong></td><td><strong>Column Two</strong></td></tr></thead><tbody><tr><td>one</td><td>two</td></tr><tr><td>three</td><td>four</td></tr></tbody></table>

^{Seen at https://madefortesting.com/wp-admin/post.php?post=512&action=edit running WordPress 4.9.8 and Gutenberg 3.7 using Firefox 62.0 on macOS 10.13.6.}

h/t @mcsf for the report at #8767 (comment)

Note: even though the alignment looks off in the editor, it looks correct on the front-end using the Twenty Seventeen theme (screenshot).

[fschtn]

I have this issues again on WP 5.9.1 😭

[talldan]

@francescosche Would you be able to make a new issue with some details of how to reproduce the problem?

If you can include information like which theme you're using, which browser and OS, that would be great. Thanks.