So in the prayers of Candlemas we read that Simeon held Our Lord in his arms, and ulna is used instead of brachium, in both the prayers blessing the candles as well as the postcommunion prayer. Any particular reason why this might be?

Brachium refers primarily to the forearm, and then more generically to the whole arm

Ulna refers to the elbow, but is used for arms in poetry and as a unit ofmeasure from embracing something (an ulna meant as much as you could grab with both arms)

The connotation is arm in connecting to hugging or embracing, as the elbows form the extremities of that holding.

Both "ulna" and "bracchium" are derived from Greek. But strange enough, Greek version uses "αγκάλη" instead in Luke 2:28. Yes, it is much closer to hugging or embracing.