M4 MacBook battery percentage drops after update

The battery's Maximum Capacity value will fluctuate up & down all the time especially if you analyze minute by minute. Some years ago I showed the math of the range of fluctuations that are typical for an Apple battery....I forget the numbers, but +/- 5% is definitely in the normal range (perhaps even a bit more). As the battery ages, those fluctuations will become even greater. It is only when those fluctuations change by 10% in minutes that there is a problem....you will probably never see this because macOS GUI doesn't poll & update the battery information that often.

FYI, the charge cycle count is irrelevant except when the value is nearing 1,000 cycles.

Just use the laptop and ignore the battery details (even if the condition shows "Service Recommended") until the battery run time is no longer meeting your expectations. Then & only then look at the battery condition & details......unless you want to observe how the battery works with these fluctuations.

All rechargeable batteries lose capacity over time. The rate of decay in "health" readings are notoriously inexact and non-linear. I see your posted data as 100% normal and expected.

How about a serving of data? This is a portion of the battery history between June 2021 and April 2023 form a 2012 Macbook Pro bought in the summer of 2013:

Carefully note how "health" plummeted over two months yet recovered. Today, the computer with its original battery is now retired but connected. Its battery shows 84% health and still delivers a very useable runtime between rechargings. And that Mac predates the more intelligent battery management systems of newer macOS version.

Are you wishing to develop a "worry ulcer" obsessing over something so imprecise and non-linear?

I hope Apple will take this seriously and restore my confidence

Apple does not participate here. This is a user-to-user community in spite of the name.

A drop from 100% to 98% in nineteen months is not out of the ordinary. There is nothing to fret about here. Your battery is behaving normally at this point and you are likely to get many years of productive use from your Mac.

The battery health drop rate is not going to be linear, and battery health may even increase from time to time.

Here is some of Apple's guidance regarding battery performance, health and management:

Batteries - Maximizing Performance - Apple Support

About battery health management in Mac laptops - Apple Support 

In a nutshell, to get the most from your battery:

• Let the OS and charging circuits manage the charging.

• Plug in when using the computer near available power.

• Use on battery when ac is unavailable or when you must be mobile.

• Don’t subject the computer and battery to extreme temperatures.

• Don’t allow the battery to regularly discharge deeply before plugging in. Yes, it may happen from time to time, just don't make it a habit.

• Don’t obsess over battery life and/or health.  

❝Worry is the misuse of imagination.❞ – Zadra