https://github.com/ankitects/anki/releases/tag/26.05b1

rename_tag in qt/aqt/browser/sidebar/tree.py optimistically updates item.name/item.full_name before the backend validates. When validation fails (e.g. a name with a space), CollectionOp‘s default path shows the error but doesn’t roll the UI back, so the sidebar keeps displaying the invalid name while the stored tag is untouched.

https://github.com/ankitects/anki/pull/4718