This isn't keto specific, but more general to weight loss as a whole. In my experience, my face gets gaunt quickly while I'm losing (and I hate it), but once I transition to maintenance there's a certain amount of redistribution/recomposition that happens all over my body including my face. Part of that is based on water retention.

How your face responds will depend on whether you just want to use keto to lose 10-20 lbs and then transition off of keto, or if you plan on sticking to keto as a long-term lifestyle once you've reached your goal weight. If you're sticking to keto long term, you'll hold less water weight long term and your face may stay slimmer. If you transition back to a diet with carbs after weight loss, your body will hold more water weight at that point and it will help plump up your face.