Is it normal for iPhone 15 Pro Max battery to degrade so rapidly?

You were really, really lucky to get a device whose battery exceeded the manufacturer’s minimum capacity. You weren’t as lucky with your iPhone 15.

A battery is a chemical device, and chemistry is generally pretty variable and uncertain, as well as being nasty analog, not nice digital. Apple specs the battery capacity to remain above 80% for 500 full charge cycles (1,000 for iPhone 15 pro max and 16), but that is a minimum requirement; there is no published maximum expected capacity. So sometimes batteries will perform much better than that minimum specification, and sometimes the change in maximum capacity won't be linear. There is no way to predict in advance what the real-life performance of any specific battery will be.

All iPhones have a specification for the battery. As an example, for the iPhone 14 Pro that is 3200 milliampere-hours (MaH). So the battery monitor is calibrated for 100% at that value. But there are variations in manufacturing, so some batteries will have less capacity, and some will have more. Suppose your battery had, say, 3520 MaH capacity (10% over standard). That would still show as 100% (even though it was actually 110%), but as it aged the health would stay at 100% until it fell below 3200 MaH. This would appear to you as if the battery had fabulous life, until suddenly it didn’t.

Batteries, like people are individuals. No two people age exactly the same and batteries don’t age the same either. The exact usage, chemical makeup, storage conditions, exposure to heat and cold and about a dozen other factors mean that every battery ages differently. Battery aging is not a linear (straight line) and degrades at different rates as the chemicals in the battery age and other factors mentioned above. The graph below shows the 4 batteries that were recharged differently and look how differently the batteries capacity changed with age. There is nothing linear in the aging process.

Apple expects your battery health to be at 80% after 24 months. If your battery health misses that expectation, contact Apple Support.

Please review the Apple Support articles on battery optimization. It contains the best  current practices for battery charging.

About Charge Limit and Optimized Battery Charging on iPhone - Apple Support

iPhone Battery and Performance - Apple Support

Batteries - Maximizing Performance - Apple

About the battery and performance of iPhone 11 and later - Apple Support