We’ve been experiencing this problem since the release of Gutenberg as well. Weirdly, it has persisted despite our having installed and activated Classic Editor, and no version up to 5.8.1 has fixed it either.
(Note: Our WP is a multi-site/network installation, and Classic Editor is network activated rather than activated individually per site. Not sure if this would make any difference, but since it’s an unusual configuration I figured it was worth mentioning.)
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