I kneel at the start of the Agnus Dei in the N.O. Too. I don’t want to wave at anybody while Jesus is out.

A. For the US version of the current missal, #43 in the General Instruction provides:

43. The faithful should stand from the beginning of the entrance chant, or while the priest approaches the altar, until the end of the collect; for the Alleluia chant before the gospel; while the gospel itself is proclaimed; during the profession of faith and the universal prayer; and from the invitation, Orate, fratres (Pray, brethren), before the prayer over the offerings until the end of Mass, except at the places indicated here below. The faithful should sit, on the other hand, during the readings before the gospel and the responsorial psalm and for the homily and during the preparation of the gifts at the offertory; and, if appropriate, they may sit or kneel during the period of sacred silence after Communion.

In the dioceses of the United States of America, they should kneel beginning after the singing or recitation of the Sanctus (Holy, Holy, Holy) until after the Amen of the eucharistic prayer, except when prevented on occasion by ill health, or for reasons of lack of space, of the large number of people present, or for another reasonable cause. however, 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 (Lamb of God) unless the Diocesan bishop determines otherwise.

For the sake of uniformity in gestures and bodily postures during one and the same celebration, the faithful should follow the instructions which the Deacon, a lay minister, or the priest gives, according to what is laid down in the Missal.

B. For the TLM, there is this:

Thanks Liam for posting. Not looking for essays as much as documented rubrics.

And the rub occurs when a priest goes to the tabernacle WHILE the Agnus is sung.

So we all fall on our knees when the tabernacle is open, but we have been excoriated by some who think we should not do so

Yes, concerning the TLM I believe the only rubric is to stand during the gospel. But I suppose that doesn’t hold true for the NO.

To paraphrase (I think) Fr. Rutler, if one has to explain to a Catholic why one should kneel in the presence of the Blessed Sacrament, he has already lost the argument.

Yes.

DURING the fraction rite (OF) no one has any business to be opening the tabernacle. I am usually saying or singing the Agnus so I remain standing. But, reflecting on this, since the consecrated elements are already present on the altar I see no particular to change my posture because of the reserved Sacrament.
After communion I kneel while the remaining elements are on the altar, and if they are reposed in a visible tabernacle until the tabernacle is closed, then I sit.

Yet it happens often… during the N.O. very often.

It's a thing much complained of by liturgically minded folks: in the OF, recourse to the tabernacle to administer Holy Communion is normatively expected to be the exception, not common, because the norm is for the faithful by and large to receive elements consecrated at the Mass they are attending, and the Fraction is ritual moment during which no minister should be doing anything else. (I and others have been in parishes where recourse to the tabernacle was uncommon at least at weekend Masses; it's quite possible to achieve.) Then again, there are things people do on autopilot: another one of those would be waiting for the celebrant to receive Communion before beginning the communion chant, when the instructions are explicitly otherwise (that is, the chant should begin *while* the celebrant receives Communion.)