More block: Undo does not work reliably for the custom text

[carolinan]

Description

In the more block, the capitalised READ MORE text in between the dashed lines can be edited.
If you edit the text and use the undo command or button, the text change is not undone.

Step-by-step reproduction instructions

Add a more block to a post or page.
Place the cursor anywhere inside the READ MORE default text, and enter a new text.
Save.
Try to use the undo button. The text change is not undone.

Test two:
Add a more block.
Customize the text.
Add two new paragraphs.
Use the undo button 4 times:

1. The most recent paragraph is undone, which is correct
2. The first paragraph is undone, which is correct
3. There is a small shift in the layout of the read more block. This is not correct. The text change is meant to be undone.
4. The more block is removed. This undo step happens without the text in the block being undone.

Screenshots, screen recording, code snippet

No response

Environment info

WordPress 6.4.3 without Gutenberg active

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