We have been doing seasonal temporary forwards for about five years. Some years it's a flawless process, some years it's a disaster. The process is handled by the USPS computer system, and yelling at your local postmaster is a vast waste of time. They can do little but check that your start/stop and forward info. is correct. One huge misconception is that your local "home" post office forwards your mail. If the system is working properly (and for us it typically is) a piece of mail, addressed to your permanent address, is diverted as soon as it hits the system, it should not travel from the sender to your permanent address, to your forwarding address.
One year, in the recent past, we had roughly three weeks prior to Christmas where we received zero forwarded mail. This included everything from a lot of Christmas cards to bills, and other important stuff. I then discovered that it's a vast waste of time to try to do anything but confirm that all the data is correct in the system. Obviously, a lot of my mail hit the trash, but USPS employees will vigorously deny that this is a possibility at all, claiming that it's a crime, and NEVER happens, which is BS. The best defense to all of this it to avoid the post office as much as possible by making everything in your life as paperless as possible. Good luck.