You must've flown into an inversion layer.
Here's what I saw this morning...
FL225: Mach .70, TAS 431, SAT -22C
FL275: Mach .68, TAS 412, SAT -33C
FL310: Mach .79, TAS 470, SAT -42C
FL350: Mach .79, TAS 462, SAT -49C
FL370: Mach .79, TAS 451, SAT -54C
The speed of sound measured by the Mach meter went from 616kts at FL225 to 571kts at FL370.
In a climb at constant Mach, decreasing TAS, decreasing SAT (until the stratosphere).