Q: The 1984 Ceremonial of Bishops describes some actions in more detail than the Roman Missal. For example for the Sign of the Cross the fingers are to joined and held straight (CB 108, footnote 81). Bishop, concelebrants and ministers keep their hands joined when walking or standing, unless holding something. (CB 107). The meaning of “hands joined” is explained in more detail, including right thumb crossed over left thumb (CB 107, footnote 80). Carrying candles for the Gospel procession, the acolyte on the right has his left hand at the base; the acolyte on the left has his right hand at the base. (CB 73, footnote 67).
In 1990, say, it was clear that Ceremonial of Bishops had the latest instructions that should be followed. But with the third edition of the Roman Missal, introduced in English in 2011, this is not as clear. The updated GIRM could have included the details above, but did not. GIRM 112 has: “At a Mass celebrated by the Bishop or at which he presides without celebrating the Eucharist, the norms found in the Caeremoniale Episcoporum (Ceremonial of Bishops) should be observed.”
It could be argued that it is impractical for ministers to do things differently when a bishop is at a parish for a confirmation; therefore the right thumb needs to be over the left thumb for all Masses.
But an approach of “do what is says; do not do what is does not say” suggests this is imposing something that is not required, for a Mass without a bishop.
To end with a question: should all ministers have the right thumb over the left, when standing in a Mass, unless they are holding something?
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Q: As you point out, the GIRM does not make these expectations as explicit as the CB does, so it does seem to leave open a door for interpretation and preference.