Description

When I remove the Header in a template and try to add a new header, I can't type forward slash header. I tried it on a custom template, but also tested it on the Page no title template

Step-by-step reproduction instructions

1. Use the Twenty Twenty-Five theme
2. Go to Templates and add a custom template
3. Select the pattern
4. Delete the header
5. Select the Content block and select 'Add before'
6. You will notice that you can't use the slash command
7. If you click on the plus icon you can select a block

Screenshots, screen recording, code snippet

Environment info

• WordPress 6.7, latest version of Gutenberg
• Chrome
• MacBook Pro

Please confirm that you have searched existing issues in the repo.

• Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

• Yes

Please confirm which theme type you used for testing.

• Block
• Classic
• Hybrid (e.g. classic with theme.json)
• Not sure