Reset does not rollback firmware. Sounds like an HDMI CEC settings issue which you can turn off. I also have a 2015 Marantz. I also run into an issue where it assumes I have ARC off my TV (which it doesn't) and it will always switch to TV input. It was just a matter of turning that feature off.
I don't blame you if trying to understand what happens when you turn CEC on is confusing. I love it and enjoy it when it works, but not all my equipment supports or fully supports it. When it does though... it's freakin' magic and I want it on. See a youtube or new episode off HBO GO off the iPad and want it on the TV. Tap the chromecast icon, then like magic. TV turns on, correct inputs chosen. Depending on what you choose your receiver turns on too or stay off. Again, all depends on the settings you choose. Luckily, but confusingly every input has it's own set of HDMI CEC settings.