I think that's exactly what it is. My iPhone 12 Pro Max charges on it perfectly when I have no case on, and I can get it to line up and charge in other orientations, even with Apple's leather MagSafe case, but I suspect the charging coil is just a little bit too low for the "Max" sized iPhones to line up when they're in cases that shift them up a little bit higher than normal; I had the exact same problem with my iPhone 11 Pro Max when I put it into a Catalyst waterproof case, but it seemed to work fine in most slim cases. I also have the exact same issue with the Twelve South PowerPic I keep on my living room table.Same issue - annoying - but turn the phone upside down and it works (must be the position of the magnets)
The iPhone 12 Pro Max obviously has the charging coil a little higher than its predecessor.