The deacons remain kneeling from the epiclesis to the elevation of the cup" (Ceremonial of Bishops (CB),155). However, the General Instruction of the Roman Missal (GIRM) states that during the Eucharistic Prayer the deacons remain standing. Which is it?
You are correct in what you have found as an "inconsistency." To ellaborate further, the General Instruction to the Roman Missal states:
During the Eucharistic Prayer, the deacon [remains] standing near but slightly behind the priest, so that when necessary he may assist the priest with the chalice or the missal (134).
Notwithstanding, the Ceremonial of Bishops states:
The deacons remain kneeling from the epiclesis to the elevation of the cup (155).
In the introduction to the CB it is stated that, "The greater part of the liturgical laws contained in the new Ceremonial have their force from the liturgical books already published. Whatever is changed in the new Ceremonial is to be carried out in the manner the Ceremonial prescribes."
Yet, custom has more precedence than law sometimes, and the custom in the United Stateshas been for deacons to stand. A particular bishop may enforce the CB rubric, because afterall, the bishop is the legislator of his diocese.