Keyboard battery level indicator

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Is the the model with replaceable AA batteries? My original Magic keyboard does the same thing. It stays on one %age level for a long time, the drops a bunch to like 50%. I've had the keyboard in constant service since 2011 and that behavior has never changed.

My Magic keyboard 2 (rechargeable) from 2018 shows a normal falloff in charge.

You may have noticed that this little foible had not stopped me from getting 12 years of reliable service from it, and it is still going strong.

I ignore it.

For keyboards and mice running on replaceable batteries, those charge remaining indicators are very inaccurate until they get very low, perhaps below 5% or 10%.

Honestly, why worry about it if all is working as you want it to? The power remaining is an estimate only and if that estimate is off, and the Mac operates normally, what's the real issue?

Apple does not do preventive or anticipatory repairs. When something fails, Apple may get involved as an paid repair or a warranty adjustment.

We all should have a standby USB mouse and keyboard. If one of the two stops working because the battery has been depleted or some other reason, you have the wired versions. My wired mouse sits on my iMac's stand 24/7; KB is in a drawer three feet away. I think I paid less than $20US for the pair.