We ran into this problem when we started snowbirding about ten years ago. If you use the free mail forwarding, you'll get 1st class. If it says "return service requested" (or words to that effect) or"do not forward" , you won't get it. It will be returned to sender. You should ask your post office about this.
We missed some doctor bills and some tax documents. You should have received 2 months of magazines, though.
The second year, we did the premium mail forwarding. There's an application fee and a weekly fee. It's pricy, but we got everything. We only use this when we stay in one place.