From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from
romanrite.com/girm.html :
“42. … A common posture, to be observed by all participants, is a sign of the unity of the members of the Christian community gathered for the Sacred Liturgy: it both expresses and fosters the intention and spiritual attitude of the participants.”
So if it is time for one person to stand and go to Communion, it is time for everyone to stand.
Unfortunately the practice in many places is for some to stand, some to sit and others kneel.
Not a good approach for unity.
Not good for singing the Communion hymn.
Not good for people trying to prepare for Communion, while wondering what posture they should be in.
Not good for those sitting, wondering if they are interferring with those kneeling behind them.
Not good for those kneeling, with someone sitting close in front of them, wondering if they should sit as well.
Not good with those kneeling at the back having their view blocked by those standing in front.
Not good because of all the distracting judgements people make about others choosing different postures.
Don’t put up with it. Stand for it. If people are free to choose the wrong posture, feel free to choose the correct posture: standing.