Keyboard battery charge percentage is inconsistent when charging with Mac

That sounds more like the displayed battery level wasn’t refreshing than the keyboard actually jumping from 67% to 78% instantly. Disconnecting and reconnecting it can cause macOS to read the battery level again.

I’d leave it connected and switched on for a while, then check Control Center > Bluetooth. If the percentage continues to increase and the keyboard otherwise works normally, I wouldn’t be concerned about the one inconsistent reading.

Apple notes that Magic Keyboard charges while connected to the Mac and recommends checking its charge level from Bluetooth:

Recharge the built-in battery in your Apple keyboard, mouse, or trackpad - Apple Support

If it keeps showing large jumps or stops charging altogether, which Magic Keyboard model do you have — USB-C or Lightning?

Yes — that’s the place Apple recommends checking it.

Since the percentage changes only after you disconnect and reconnect the keyboard, that makes a stale battery reading more likely.

I’d leave the keyboard switched on and connected for another 30–60 minutes, then unplug and reconnect it once. If the percentage jumps up again, the keyboard is charging and macOS just isn’t refreshing the displayed level properly.

I wouldn’t reset Bluetooth or change anything else yet. If the percentage stops increasing even after reconnecting, let us know whether it’s the USB-C or Lightning Magic Keyboard.