Why does my tablet battery not drain past if it is plugged into the keyboard?

[ma ke]

I have a Surface Book 2 and it usually would have both batteries drain their percentages at the same time when plugged in together but recently the tablet battery doesn't drain when it is plugged in with the keyboard but the keyboard battery keeps draining.

Nevermind. When my keyboard battery got to 40%, the tablet battery started draining instead while the keyboard battery stayed at 40%.