Fitter error. They should refit the whole enclosure, tray and all, and be very apologetic about it. Hope that's how it plays out for.you. Good luck.
Just to back up what you've said - it looks like on one side they've fitted the tray onto the walls studs. Then done plasterboard and tiles, meaning too much of the tray is lost behind them, obviously leading to the enclosure not having enough space.
He'll no doubt blame the two workmen he had working for him who did most of the work, he later told me he'd sacked them for botching stuff... but he should have noticed this himself. Having said that he probably did. He said he did the sealing after the enclosure was fitted, so surely any competent fitter would have noticed that the enclosure was sticking out from the tray! He probably just prayed I wouldn't notice and there would be no issues.