From Genesis (of course there would not be a shortage if the US had not attacked Iran):

President Donald Trump explicitly asked Ukrainian President Volodymyr Zelenskyy to halt strikes on Russian oil refineries and diesel infrastructure.
Business Today

Speaking to reporters in Doonbeg, Ireland, Trump stated that he had directly spoken to Zelenskyy, saying: "Mr. Zelensky has to do one thing. He has to stop knocking out diesel fuel in Russia... there are plenty of other targets. Don't hit diesel fuel, because that's hurting the world."
AP News

Key Drivers Behind the Request
Global & U.S. Energy Shock: Ukrainian drone strikes targeting Russian refineries have significantly curtailed Russia's diesel production and exports. Combined with Middle East supply disruptions, this has driven U.S. diesel prices past $6.00 per gallon.
PBS

Domestic Economic Pressure: Elevated diesel prices raise transportation, shipping, and agricultural costs, keeping U.S. inflation high at a politically sensitive time.
AP News

Leverage Over U.S. Aid: The U.S. remains Ukraine's primary source of intelligence and military hardware, giving Washington substantial leverage to press Kyiv on specific target selections.

Plausibility of Ukrainian Compliance
Strategic Angle Realistic Response
High Strategic Value Long-range drone strikes on refineries are one of Kyiv's most effective asymmetric tactics, cutting into Russia's war revenue and causing domestic fuel shortages. Giving this up without major concessions is military disadvantageous for Ukraine.

U.S. Political Leverage Ukraine relies heavily on U.S. air defense interceptors, intelligence, and financial backing, making it extremely difficult for Zelenskyy to completely ignore direct requests from the U.S. administration.
Potential Compromise Ukraine may pivot to targeting alternative military and supply chain infrastructure—such as ammunition depots, rail lines, or air bases—to respect Washington's line on energy markets while maintaining pressure on Russia.

While Kyiv views Russian refineries as legitimate military-economic targets, Trump's explicit demand creates immense pressure on Ukraine to shift its targeting strategy away from refined petroleum products.
AP News

Jeff