Did Jiffy Lube or Tire Kingdom flush the clutch fluid? Or did they flush the manual transmission fluid?
A manual transmission doesn’t get flushed, you just drain the old fluid out and refill with new transmission fluid.
If the hydralic fluid was flush for the clutch system they may have refilled it with the wrong type of fluid. It seems you loose pressure when the clutch isn’t depressed for a long period of highway driving. This can be a leaky master cylinder, or leaking slave cylinder, but more likely the master because it is the part of the system generating the pressure. The wrong fluid could be getting hot and developing air bubbles.
You need to take it to either a good Mazda dealer or a good independant transmission shop. Stay away from Kingdom tire and Jiffy Lube for service period. It is not the norm for a master cylinder to fail at 56K miles, but it happens often enough that this could be your problem.