First of all, it would be best that you contact the editors of NAB and ask them to explain their choice. I can only interpret the rationale; I cannot explain their decision.
I believe it is a choice of interpretation. For many, “Holy Spirit” together is considered to be the proper name of the Third Person of God. However, the Holy Spirit also has many “names”: Spirit of God, Spirit of Truth, sanctifying Spirit, Advocate, Paraclete, or just Spirit.
The editors of NAB apparently has chosen to interpret “Spirit” alone being the proper name, and “holy” merely as a qualifying adjective to distinguish Him from just any other spirit.
However, in Catechism 691, it says clearly that “Holy Spirit” together is the proper name.
So, somehow NAB missed that. I consider this a gloss, and not a serious mistake. After all, Spirit is one of His names, and it is certainly not meant to be irreverent.