it subsequently went supersonic well before the X-1
The first flight of the F-86 to be monitored by NACA recorded Mach 1.03 on 21 November 1947. Welch had been flying the same profile on the days prior to the X-1 flight, although they were unmonitored. The X-1 flight was annotated "the first in level flight". Recognition perhaps that they were not the "very first"? Officially the 86 didn't make it until 26 April 1948.