The currently used #prefix and #suffix at the $build[$region] should be avoided. This practice has the possibility that something goes wrong (mistyped/accidentally missing opening or closing tags) and it's impossible to add new attributes (even classes) to the wrapper element without setting #prefix again. If somebody doesn't need the wrapper, he/she has to unset #prefix and suffix too.
Use #theme_wrappers instead of the current solution
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My related comment for Panels module
https://www.drupal.org/project/panels/issues/2820617#comment-15243637
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StandardDisplayBuilder::buildRegions should use block_list theme, not suffix/prefix
Needs review