If I use the prefix "wdtn:" in the SPARQL engine on https://query.wikidata.org/ , the interface complains that wdtn: is an unknown prefix.
To reproduce go to https://query.wikidata.org/#SELECT%20%3Fitem%20%3Fulanid%20%3Fulanuri%20WHERE%20%7B%0A%20%20%3Fitem%20wdt%3AP245%20%3Fulanid%20.%0A%20%20%3Fitem%20wdtn%3AP245%20%3Fulanuri%20.%0A%20%20%7D%20LIMIT%2010%0A%20%20%20%20%20%20%20%20 , click the wdtn: and do CTRL-space
The expected behavior is visible when you hover over the wdt:P245, you'll get "ULAN ID (P245) identifier from the Getty Union List of Artist Names"