A sudden drop in appetite after moving from working from home to daily office hours is common, usually tied to stress, disrupted sleep and eating at odd times rather than anything serious. The mouth issue slowing your eating is worth checking separately, since chewing discomfort itself can put you off food.
Next Steps
Anchor breakfast and lunch to fixed times, even if small, and stick to foods you enjoy, like your muesli with milk or banana shake, instead of forcing dry options. Get the mouth issue checked by a dentist, and see a physician if poor appetite continues past two weeks or you lose weight.
Health Tips
See a doctor sooner if you develop fatigue, pain while swallowing, or noticeable weight loss.