3 months after brake change in workshop: brake disc annealed and surface melted

Hello guys, I need your swarm knowledge again! The following scenario: I had my brakes on the front axle made anew about 3 months ago (ply + disc). They were already pretty blank and squeaked. In the workshop then changed, as far as everything was good. Now I did a 2h tour with mountain passages in the currently hot weather. Suddenly it started to stink and smoke. Directly stopped and noticed that it comes from the front left brake system. No F The tyre pressure was already at 350 kPa due to the heating compared to the 290 of the other tyres. I put the car in the same workshop today and described the events. They examined it and said that it comes from a clogged/squeaked brake hose. The manchet on the brake piston has melted and it has to be completely overhauled. The brake disc and brake blocks are och ok (?!). To my question: Wouldn’t that have had to be noticed during the change three months ago (e.g. when the brakes are ventilated)?. Is it just an excuse because a faulty assembly caused the overheating (was also only at the front left) ? Can I still drive with the brake disc and pads (burned/glazed)? Thanks already! LG JM