The kneeling is a deviation, because the Church does not say “that people can indeed kneel or sit or stand when they return from receiving Holy Communion”.
Here is what the Church does say: “… and, as circumstances allow, they may sit or kneel
while the period of sacred silence after Communion is observed.” (From the 2002 General Introduction to the Roman Missal (GIRM) which can be accessed from
romanrite.com/girm.html ).
It wrong to suggest that the “period of sacred silence after Communion” begins when there is singing. There is not silence if the priest is saying “The body of Christ” to people.
As I posted above, the Ceremonial of Bishops is indicating that everyone sits when the bishop sits, for his Stational Mass.
“166 When the bishop returns to the chair after communion, he puts on the skullcap and, if need be, washes his hands. All are seated and a period of prayerful silence may follow, or a song of praise or a psalm may be sung.”
Ceremonial of Bishops, Liturgical Press, 1989, ISBN 0-8146-1818-9, page 60.
The Latin title of this book is Caeremoniale Episcoporum.
Its instructions remain relevant, even though the GIRM has been published more recently, because the 2002 GIRM includes:
"112. … At a Mass celebrated by the Bishop or at which he presides without celebrating the Eucharist, the norms found in the Caeremoniale Episcoporum should be observed.
Everyone having the same posture in this way is consistent with 2002 GIRM n. 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.”