Sudden 7% battery health drop in new iPhone 15

Battery health decline is not linear, meaning it can remain steady for a while, then drop a few points quickly. At 256 cycles, it would be expected your phone would have lost from 5 - 6% in health by this time. So a 7% drop is certainly nothing to be concerned about. Your health could actually be 93.9%, closer to 94%, but the health reading is only in whole numbers, not fractions.

You don't have an issue with your phone. And Apple warranty wouldn't be applicable to your battery unless its health dipped to 80% or less in the first 12 months.

Things you should be doing:

1. NEVER EVER let the battery fully discharge - it can prematurely age the battery.
2. Avoid going below 20% - same as above.
3. DO plug your phone in to charge when you go to bed at night, with Optimized Battery Charging turned on and leave it on its charger ALL night, EVERY night. Make sure to have Wifi turned on so you backup every night to iCloud as well.
4. Read this about Maximizing Battery Performance --> Batteries - Maximizing Performance - Apple
5. Don't try to micromanage your battery or be constantly checking its health. Batteries - ALL batteries are consumable parts, which lose their ability to hold the same charge they did when new with every full cycle charge.
6. And finally, when the health of your battery dips to 80%, which is a long way from now, that's when you pay Apple to replace the battery in your iPhone.

A decrease that average 1-2 percent each month is normal.