Another update (12/4/20):
I just spent over 3 hours with at least 4 different Apple support technicians on the phone last night. The last senior advisor basically said that Apple has not officially acknowledged that there is a wide-spread problem with iPhone 12 and 12 Pro phones having wireless charging issues as "these phones are relatively new". Not sure I can accept that as they had to have had a long QA period to fully test these. I pointed them to this forum and none of them showed much interest or could validate the accuracy of the posts. Really?
Rather, they made me go through all of the known steps that most of us have reported here (reboot phone, hard reset, ensure latest iOS update is installed, try different chargers, make sure the power supply plug is 20W, remove cases, try different positions on the chargers). NONE of these of course worked with my phone. The final suggestion was to do a full encrypted backup to a Mac, reset the phone to factory default using the plug and hold button procedure, and then do a full restore from the encrypted backup). And that did not make a difference either (as I expected).
So, now they recommended that I call back and submit a request to have a NEW replacement phone mailed to me and they will put a hold on my credit card until I send the other one back. I'm not confident this will fix the problem either but maybe there is just a bad "lot" of phones that got shipped out with this hardware problem? None of the tech support people could confirm that a future iOS update would fix the problem, and even that won't happen unless Apple development can see that it is a widespread problem. This will only happen if ALL of us reports this to their tech support.
I also got the official Apple MagSafe charger with their 20W adapter and it too is a little glitchy when charging. I have to position the iPhone 12 Pro Max in EXACTLY the perfect position or it won't charge. And while it is charging, if it gets bumped or nudged even a slight bit then it stops charging. My wireless car charger, and 4 other wireless chargers in my home office, bedroom, and family room all work fine with my family's other iPhones (X, XS, and 11 Pro) -- all with cases and all charge instantly when placed on the chargers. My 12 Pro Max fails to charge on all of them (all different brands).