Thanks for the report! The theme should be corrected, even if a change to the editor would fix the problem.
Some themes set the Separator block max-width to 100px without making an exception for the Dots style, which had worked with earlier versions of Gutenberg. Then GB30255 switched max-width: none to width: none, and GB33793 removed that for being invalid.
Twenty Fifteen already adjusts the max-width for Wide separators:
.wp-block-separator.is-style-wide {
max-width: 100%;
}
And it could have the same for the Dots style:
.wp-block-separator.is-style-wide,
.wp-block-separator.is-style-dots {
max-width: 100%;
}
Twenty Fifteen would need edits to both blocks.css and editor-blocks.css.
Other themes would need a similar change in only the blocks.css file (for the front end):
- Twenty Eleven
- Twenty Thirteen
- Twenty Fourteen
- Twenty Sixteen