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Thought this might be the same bug asbut I just checked and conditional_fields doesn't use hook_node_validate() to do its thing, so this is a separate bug.
NodeInlineEntityFormController overrides entityFormSubmit() to invoke node_submit() and friends, but it doesn't do the same for node_validate(). This breaks modules like unique_field and is generally an extremely dangerous thing to do. Sites might bake all sorts of validation into their nodes, and if that can be totally bypassed via IEF, badness ensues. ;)