The two words have very slightly different connotations. "Formally" is neutral in the sense that it implies that whatever it may be was proposed in accordance with some established procedure. So, for example, you could not say, other than in jest, at a restaurant meal with friends "I formally propose we should choose the soup". But a meeting of your local bird-watching society could formally agree to meet more frequently.
To use the word 'officially' implies that the activity is not just procedural but that the procedure flows from some authority (or 'office'). That is slightly less neutral than 'formally' because it suggests that the writer recognises that authority in some sense. So, for example, the Mayor could officially ban meetings of the bird-watching society, but the bird-watchers could only protest formally (because they have no official position).