A recent U.S. Supreme Court decision to review a case called Fischer v. United States, which experts say could weaken prosecutors’ hand in hundreds of Jan. 6 cases, including former President Donald Trump’s, is already upending some defendant cases and sentencing proceedings.In December, the Supreme Court decided it would take up the appeal by Jan. 6 defendant Joseph W. Fisher of the Biden administration’s novel use of an Enron-era evidence-tampering law to prosecute hundreds of defendants for obstruction of Congress during the Jan. 6, 2021, Capitol incident.The obstruction of Congress charge—which carries a sentence of up to 20 years in...