The Quran describes Dhul Qarnayn's journey:

“And he followed a road till, when he reached the setting-place of the sun, he found it setting in a muddy spring, and found a people thereabout.”
Quran 18:85-86

The full story goes like this:

The Dhul Qarnayn story begins: “They will ask thee of Dhu’l-Qarneyn.” In it, he traveled until he reached (balagha) the setting place of the sun and found (wajada) the sun setting in a muddy spring, and found (wajada) a community near it. Then he reached (balagha) the rising place of the sun where people had no shelter from it. Then he followed another path until he reached (balagha) a place between two mountains, where he found (wajada) a people asking for help against Gog and Magog. He built an iron and copper wall between the cliffs to seal them off until doomsday.
Quran 18:83-102

Quran 18:86 in Arabic word for word creates two independent problems:

1. First, Dhul Qarnayn reached (balagha) a location: the setting place of the sun (maghriba l-shamsi).
2. There, he found (wajada) the sun setting in a muddy spring (ʿaynin ḥami-atin). And he found (wajada) a community near it.

The earliest tafsirs, including Tafsir al-Tabari, literally said the sun set in a muddy spring:

“The sun sets in a spring of murky, black mud; then it falls down in prostration beneath the Throne, seeks permission [to rise again], and is granted permission.” (on Quran 36:38)
“This means that the sun and moon enter beneath the earth from the west and emerge from beneath the earth in the east, then travel through the sky until they set in the west. This is their orbit.” (on Quran 36:40)
“‘He found the sun setting in a muddy spring’ - that is, a hot, black spring.” (on Quran 18:86)
Tafsir Muqatil (d. 767 AD)

“It has reached me that the sun sets in a spring, and the spring hurls it to the east.”
Tafsir Sa’id b. Mansur (d. 842 AD)
Tafsir al-Tha’labi (d. 1035 AD)
Tafsir al-Suyuti (d. 1505 AD)

“A city with twelve thousand gates; were it not for its inhabitants’ voices, people would hear the sun fall as it sets.”
Abu al-Shaykh, al-Azama (d. 979 AD)
Tafsir al-Suyuti (d. 1505 AD)

“The reading ḥamiʾah means [a spring] of black, foul-smelling mud; ḥāmiyah means ‘hot.’ … ‘And he found (wajada) beside it a people’ - that is, beside the spring in which the sun sets.”
Tafsir al-Samarqandi (d. 984 AD)

Tafsir al-Tabari (d. 923 AD), widely considered one of the most authoritative Quranic commentaries, on Quran 18:86

Tafsir al-Tabari preserves the debate between Muhammad's Companion Ibn Abbas and the salaf over whether the spring where the sun sets is muddy or hot. And Ibn Abbas, who Muhammad prayed for to get the correct interpretation of the book (Sunan Ibn Majah 166), said that the sun sets in a muddy spring. Tabari says both readings are correct because the sun could set in a spring that is both hot and muddy:

Allah says: (Until, when Dhul-Qarnayn reached the setting of the sun, he found it setting in a muddy spring.) The readers differed on how to read this. Some of the readers of Madina and Basra read it as (in a muddy spring), meaning that the sun sets in a spring that contains mud. While a group of the readers of Medina and the majority of the people of Kufa read it as, (in a warm spring) meaning that the sun sets in a spring of warm water.

Al-Husayn b. Al-Junayd ← Sayd b. Salamah ← Ismail b. Aliyah ← Uthman b. Hadir: I heard Abdullah b. Abbas said: Muawiyah recited this verse, and he said (warm spring) and Ibn Abbas said: it is (muddy spring). He said: So they sent to Ka'b Al-Ahbar and asked him. Ka'b said: As for the sun, it disappears in 'Thatin', which matched what Ibn Abbas said, and the word tha'at means "mud".

And in my (Tabari's) mind the correct opinion is to say that they are both popular readings in the land, and each one has a correctness about it and an understandable meaning, and neither contradicts the other, for it is possible that the sun sets in a hot spring that has mud and sludge, so a reader who uses "hot spring" is describing its temperature, and the reader who uses "muddy spring" is describing that it has mud and sludge. Both versions have been narrated to us.

Muhammad b. Al-Muthanna ← Yazid b. Harun ← The common people ← A freed slave of Abdullah b. Amr ← Abdullah: "The Messenger of Allah, may Allah bless him and grant him peace, looked at the sun when it set, and said: 'In the blazing fire of Allah, in the blazing fire of Allah: If it wasn't for Allah's command, the sun would burn all those who are on earth.'"
Tafsir al-Tabari (d. 923 AD) on Quran 18:86

They started changing it only after it became too embarrassing.

See more here:
https://islamsproblems.com/quran-sun-sets-in-muddy-spring/