Battery health dropping significantly on my iPhone 13 Pro

Well, on average Battery Health will drop about 1% a month. That's an average, not an absolute. The phone might go 2-3 months without dropping at all and then drop 2-3% the following month.

So, your phone is about 7-8 months old at this point, so 92% would be normal.

Replace the battery when the Health gets down to 80%.

There isn't necessarily "normal". There's average and that can mean everything from barely loses capacity vs premature failure. It's not a guarantee other than Apple will replace the battery under warranty if it drops below the nominal rating of 80% during the warranty (or AppleCare) period and it has recorded less than 500 cycles. 80% after 500 cycles is what Apple considers its target for battery longevity. Most users will never see that within the warranty period, although it might be possible with AppleCare.