There are two separate issues here. First, a bishop in the United States can require that congregations in his diocese remain standing after the Agnus Dei until the reception of Communion. If your bishop has mandated that the congregations should remain standing after the Agnus Dei, then you should stand as has been directed:
In the dioceses of the United States of America, they should kneel beginning after the singing or recitation of the Sanctus until after the Amen of the Eucharistic Prayer, except when prevented on occasion by reasons of health, lack of space, the large number of people present, or some other good reason. Those who do not kneel ought to make a profound bow when the priest genuflects after the consecration. The faithful kneel after the Agnus Dei unless the diocesan bishop determines otherwise. (GIRM 43, emphasis added)
Second, the bishop does not determine a congregant’s posture after the congregant has received Communion. After each congregant has received Communion, the congregant may choose to stand, sit, or kneel. The congregant does not have to wait for the rest of the congregation to receive before assuming the posture of his choice. If you wish to sit or kneel at this point, the choice is yours. In 2003, the Congregation for Divine Worship and the Discipline of the Sacraments (CDW) released a response upholding this. The question-and-answer can be read on the Web site of Adoremus (www.adoremus.org).
This response by the CDW has been thought by some to indicate that the bishop cannot mandate that the congregation remain standing after the Agnus Dei. This is not the case. The response was a clarification of number 43 of the GIRM, not a negation of it. This means that while the bishop can mandate standing until the individual reception of Communion, he does not mandate that individuals may not kneel after receiving Communion.