Unify to “亚尔”.

The name comes from གཡག་ར་གཞུང་ཆུ (Yag-ra-xung-qu, gyag-ra-gšuń-cʰu by myself). Yag for ‘male yak’, Ra for ‘goat’, Xung for ‘main’, Qu for ‘river’. The correct translation should be ‘亚克惹雄曲’, ‘亚尔惹熊曲’ is a just-so-so translation. But ‘亚尔’ is definitely not a character.

We have established contact with the Department of Division and Toponym of the Department of Civil Affairs of the Tibet AR, and here is a reply.



(Translation by myself)
Hello!
I am 张建明 from the Department of Division and Toponym of the Department of Civil Affairs. I have received your message on the National Toponym Information System platform regarding your doubts about the transliteration of 亚惹尔雄曲 in Tibetan language. The department attached great importance to your message and would like to express our sincere gratitude for your concern about the geographical names of our region.
Regarding the toponym 亚惹尔雄曲, we would like to clarify the following:
(1) After verifying with the Tibetan language toponym experts of our office, the toponym 亚惹尔雄曲 is correctly expressed in Tibetan script, meaning and Romanization in the platform data;
(2) The place name was provided by the census officer of Duilongdeqing County during the field census of toponyms, based on the actual census, and the translation of the Tibetan name of the people at the time of the census was retained in accordance with the principle of convention and not changing the habits of the people. Therefore, the name of the place 亚惹尔雄曲 is correct.
Hereby reply. Thank you again for your attention! At the same time, you are welcome to continue to pay attention and work together to the excellent toponym culture in our region!

The map of 堆龙德庆县， in 《堆龙德庆县志》，北京：中国藏学出版社，2010年7月

And a clearer image of the map

The evidence is made by the Survey and Mapping Institute of Tibet AR (西藏自治区测绘院), who confirmed the accuracy of the characters. The combining “尔” is used to record the er-diminutive-like ending in གཡག, which is common in 同文韵统, and the encoding model remains to be discussed.

P.S. “亚惹尔雄曲” is provided by force in the 1:50000 map, which is also correct.

The character appears in a fomal administrative map and is cited by another published book, which means it should be encoded even it's "wrong". What's more, characters actually used by people should not always satisfy academic facts. They can vary instead.