Applied for a M visa to trade show, got L type

Dear, it won’t be a problem. You can use the double-entry L type to attend the trade show. It’s a usual case. In most cases, the consular officers will issue the relevant type to the applicants according to their travel purpose. Maybe they think a tourist L type can cover your trip.
It’s not necessary for you to tell the boarder that you go there for business purpose. You can just say that you are a tourist and then you can attend the trade show.