The problem is that the amendment that passed is a weakened version of Sanders' original amendment that would have allowed for future audits as well. It is reported that the request to weaken the amendment came from the White House as well as other sources.
Open Congress notes how Senator David Vitter (R-LA) sought to revive the tougher version:
Thanks to Sen. David Vitter [R, LA], something very similar to the original Sanders amendment is going to get a vote today as well. In addition to removing the special protections from audits for the Fed from U.S. code, the Vitter amendment adds new language to help preserve the Fed’s political independenceOpen Congress also notes that Tom Harkin (D-IA) and Ben Cardin (D-MD) were both co-sponsors of the original Sanders amendment. The both voted no on the Vitter version.
Ron Wyden (D-OR) was also a co-sponsor of the original and voted yes on the Vitter version.