The Arbutus theme as a number of incompatibilities with the gallery block refactor as outlined here.

• .wp-block-gallery.blocks-gallery-grid.has-nested-images figure.wp-block-image img { width: auto; } breaks the layout/widths for some reason
• Custom theme styling for captions is completely missing because the markup and selectors are different. (this applies to all themes tested)
• This theme styles captions to not overlap images, because it’s designed to work with longer captions.
• Theme authors should not need to make any changes to accommodate existing use cases with the revised gallery implementation. It would be reasonable to require themes to update to allow the new features that the change enables.
• Custom gallery excerpts are broken. Core PHP functions get_post_gallery() and get_post_gallery_images() still don’t work, and fallbacks for current block galleries don’t work because images are missing the data-id attribute.