Is it normal for my iPhone 15 Pro battery health drop to 91% for 336 cycle count?

It's not unreasonable. One would expect after 336 battery cycles, your battery health would be close to 93%, but a point or two difference, is not that unusual. But battery health decline isn't exact, nor linear. And how you use and charge your phone can and does make a difference. For example, if you let the battery fully decline, it can reduce battery health decline faster than if you don't let it go below 20%. Do you plug your phone in to charge when you go to bed at night, with Optimized Battery Charging enabled, leaving it plugged in all night EVERY night? If you don't, that's exactly what you should be doing. The reality is you cannot stop battery health decline. When your battery health dips to 80%, it will then be time to pay Apple to replace the battery.

Read this --> Batteries - Maximizing Performance - Apple

Also read this --> When to charge your iPhone or iPad - Apple Community

The battery in the iPhone 15 Pro is designed to go up to 1,000 Charge Cycles......under ideal conditions.......before the Battery Health declines to 80%.......which works out to a 1% decline for every 50 Charge Cycles.

Unfortunately, Apple does not define ideal conditions and 5 different users might have 5 different opinions on what "ideal" might be.

Since your battery has completed 336 Charge Cycles divided by 50 = 6.72% or 7% rounded off.

100% - 7% = 93%, so your battery at 91% is a shade below target at this time. But, no need to do anything special at this point.

Remember that Apple always rounds down any tenths of a percent, so your battery might actually be at 91.9%......but this would be displayed as 91%.