Lost your rear brake because the rear wheel was misaligned and your rear brake pads were dragging on the rotor - creating heat and boiling the brake fluid.
Air in your clutch line would create the opposite situation. Clutch will not disengage with air in the clutch fluid, so the problem is not likely at all clutch related.
Failed ISO drive or missing components, failed rear wheel bearings, missing/incorrect wheel spacers installed at recent rear tire replacement. Could have stripped/damaged rear drive belt as collateral damage. Something is broken or missing allowing the rear wheel to become misaligned. Maybe so misaligned the drive belt is loosing engagement? Take it back to whoever changed the rear tire. They did something wrong. Be prepared for them to tell you it's not their fault!

And make sure they also inspect your rear brake rotor, caliper and brake pads. And flush out the rear brake fluid and replace with new. Once it boils, the boiling point drops dramatically. You will want new fluid. Not just bled, but flushed and bled.