TempDB is in simple recovery model, Backup log with truncate_only does nothing to a DB in simple recovery.
When shrinking a log file, you can't shrink past the active portion of the log. If that's right at the end of the file, you'll need to wait until it cycles around to shrink the log.
p.s. Why are you shrinking the log anyway?
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability