More details from the NY Times:

Mr. Bessent and Mr. Long had clashed at times, three [sources] said. Mr. Long had remarked to some colleagues that he had to ask Mr. Bessent for permission for everything he did at the I.R.S., two of the people said.

Others said Mr. Long had at times appeared to get out ahead of the I.R.S. and Treasury Department. He told tax practitioners last month that the agency’s all-important filing season would start later than usual next year, a statement that the I.R.S. later said was premature.

From the Washington Post:

Long will be nominated as U.S. ambassador to Iceland, he posted on social media Friday afternoon.

Long had made several changes that frustrated leaders in the Treasury Department, according to two other people familiar with the president’s decision, also speaking on the condition of anonymity to discuss a sensitive matter. He had planned to start tax filing season around Presidents’ Day in February — about a month behind schedule — and also preemptively announced plans to eliminate the IRS’s Direct File program, even though Treasury officials had not decided to do so.