That kind of makes sense what the mechanic said. You can make a gauge out of wood(1). Tape it perpendicular to a long stick(2). Set the deck at a medium height. Spin the blades so they are going left to right. Make the gauge(1) short enough to fit between the blade and the ground. Use the long stick(2) as a handle so you can slide the gauge to check each blade edge thru the grass chute.
From your description, it almost sound like the belt is losing traction. I had an MTD that did this. Turn on the deck and it would have a certain sound. As soon as I went into grass, you could hear the deck slow down. Still did it even with a new belt. I sold it to someone else as is and not sure how it was fixed. Maybe someone else has come across something like this.
Check the blade pulley on the engine. See if the belt bottoms out in the "V". If so, then the belt may not be gripping on the sides like it is supposed to. Check the idler pulleys for the belt. Make sure they are not stiff. Make sure the belt isn't threaded wrong. Inside of belt goes to v pulleys. Outside of belt goes to flat pulleys. If this is an electronic pto, then the magnet might be slipping. Or battery doesn't have enough power to give the magnet full power. If mechanical, make sure the springs have tension when engaged.