You need to scan for channels every time there’s a change in local broadcast frequencies. That means you have to rescan every time you move, every time you buy a new TV or antenna, and every time a local TV station changes to a different broadcast frequency. Due to an FCC mandate, broadcasters are steadily switching over to 4K-ready, ATSC 3.0 compatible frequencies that don’t interfere with cellular frequencies. As a result, all TVs are going to lose track of local channels slowly. Plus, entirely new channels could pop up in your area, and your TV won’t know that they’re there.
Scan for new channels every month, or every time you notice that a local broadcast goes missing. It’s an easy process, and it’s worth doing for the sake of free OTA TV.