Changing alignment of the cover block when set to fixed position breaks image

[brentswisher]

Description

Originally reported by @Azragh here. There are issues with the cover block losing the background image when some settings are changed or the block is duplicated with ctrl + shift + D.

I have verified both that this is happening for me, but that it is unrelated to the changes in #28499. (I reverted those changes locally and it was still happening) I know @jasmussen has been working through some issues with the cover block, so possibly related to those?

Step-by-step reproduction instructions

1. Create a new post
2. Add a cover block with a background image
3. Toggle the "fixed background" option
4. Change the alignment of the cover to be aligned left
5. See that the background of the cover is now just the color black

Expected behaviour

That the background image remains when the alignment is changed

Actual behaviour

The background image disappears when the alignment is changes

Screenshots or screen recording (optional)

WordPress information

Latest master using local wp-env environment

Device information

• Device: Desktop
• Operating system: Mac OS Catalina
• Browser: Chrome 88.0