80% chance it’s air trapped in brake lines. (or too much moisture resulting in water vapor forming when hot)
Flush brake fluid. Make sure you do this properly – many shops ‘fake flush’ by just siphoning out brake fluid reservoir without bleeding brake lines.
Watch whether mechanics actually perform bleeding at all 4 wheels until new, clear, fluid comes out. Demand moisture test on new brake fluid and don’t leave shop until test comes out right.