Vehicle model years are based on what is being produced on January 1st of the year--so a 2022 model year can start anytime after January 1st, 2021, and then continue to any point before January 1st, 2023. When any given vehicle manufacturer decides to make the changeover from one model year to another during the calendar year is entirely up to them. It's also possible to skip a model year entirely even with more or less continuous production if the changeover is taking place around the new year.
For trailers, I'm not sure they have model years in the same sense that cars and trucks and other self-powered vehicles do--model years that the government cares about--so maybe the standards are a little more loose with them.