To reproduce:
- Set your language in Wikidata to English.
- Go to an item that doesn't have a label in English (example query)
Instead of a label, the words "No label defined" are shown (it's the wikibase-label-empty message).
It would be more useful to show some label than to show nothing at all.
There is a related bug, which was marked as resolved: T88193: Create label fallback mechanism. But the problem, as described in this bug, still remains.