I just got the iPhone 12 Pro Max yesterday and noticed the exact same thing with all 3 of our wireless chargers (different brands and models). One of the Qi stand models will work only if I put the phone on its long edge (landscape), but not in the standard portrait orientation. The other two chargers won't work at all. All 3 chargers work fine with my previous iPhone 11 Pro Max.
I spent 30 minutes with Apple tech support and they not only could not confirm there was an issue, but said to bring the phone into an Apple Store for them to look at it! So they obviously did a TERRIBLE QA on these Pro models and don't care that users won't be able to use most of their wireless chargers at home, office, or in the car, as well as leverage any that are in cafes and public places. THIS IS A HUGE BLUNDER!
Anyway, couldn't get tech support to acknowledge the problem so obviously they wouldn't offer any advice or information regarding a possible software fix in a future update (assuming it can be done). If this is a problem with the hardware and its circuitry positioning, then they will have a lot of unhappy customers.
The 12 Pro Max arrived yesterday with 14.1 so I updated it to 14.2, also also tried the full reboot like others have suggested, but nothing has helped.