I have encountered the same problem.
The content of the Cover Block appears on right side of the background image. This often pushes the background image outside of the Div.
It doesn’t help to ‘center’ everything, nor does it change if I turn off caching etc.
Also: the options to add a Title, and/or an extra paragraph + a button sometimes appear, and sometimes they don’t.
When adding a button, it jumps outside of the entire block, with no possibility to drag it inside the cover block at all.
I got the same display error
Workaround: enable “Fixed background”
I am also experiencing this issue. I’m seeing in the console:
New content generated by 'save' function:
<div class="wp-block-cover alignfull has-background-dim-30 has-ruf-11-background-color has-background-dim is-style-default has-height-lg has-width-md"><img class="wp-block-cover__image-background wp-image-847" alt="" src="http://localhost:10003/wp-content/uploads/2020/07/picture.jpg" data-object-fit="cover"/><div class="wp-block-cover__inner-container"></div></div>
Content retrieved from post body:
<div class="wp-block-cover alignfull has-background-dim-30 has-ruf-11-background-color has-background-dim is-style-default has-height-lg has-width-md" style="background-image:url(http://localhost:10003/wp-content/uploads/2020/07/picture.jpg)"><div class="wp-block-cover__inner-container"></div></div>
This happens even when I’m creating a new block, which I don’t understand. Has Gutenberg cached an old version of the block?
