HVAC Technician told us never to use an air filter that is higher than Merv 8. Is this a thing or is there something faulty with our Furnace?

Our Furnace would stop sending heat out because it was overheating. The tech came and told us it’s because our Air Filter is Merv 12 and that we should be using Merv 8.

Is this a real thing? Is Merv 12 too strong of an air filter for some furnaces or did he just not want to fix our Furnace? (It’s under warranty still)