The mystery of why the whole/entire verse was simply never quoted, (TNC)
Not much of a mystery.
First, we are dealing with the Johannine Epistles, which have limited quoting.
It is common for Ante-Nicene writers, especially, to not quote a full verse. Do some checking with the famous battleground verses like 1 Timothy 3:16, Acts 8:37 and dozens more. I have some of them on PBF because generally the apparatuses are very unreliable and also don't give the text.
This is one reason why textual criticism discussions are often shots in the dark. James Snapp has helped on a few variants, including the Mark ending and the Pericope Adulterae, also Acts 8:37 and some others.
Of course, there are exceptions, especially John 10:30.
Abundant quoting, even Ante-Nicene, helped by brevity.
It is fairly common even later that our quotes are partial.
Once you get to the late fourth century, it is very common for the heavenly witnesses verse to have a full quote. Which in terms of textual referencing is fairly early. This has been hidden by contra authors. (Which has had an effect on defender writing.) Even Grantley McDonald found ways to hide the truf of the full quoting. He wanted to nurture the theory that there was some sort of formulation process over centuries, so you will find very few of the few quotes, and those usually in Latin only. Only The Witness of God is Greater, and my summary projects, touch this properly. Nick Sayers makes an effort to keep up.
Look for full quotes of 1 John 5:8.
The pickens are slimens.
And I did some checking on 1 John 2:23b. It is true that Cyprian did give a full quote, but that was the only Ante-Nicene one I could verify. Once you get around AD 400, full quotes are common, exactly as we see in the heavenly witnesses.
One thing that is humorous is the claim that the partial quotes of the heavenly witnesses verse, referring to Father, Son and (Holy) Spirit, are actually invisible allegories of the spirit, water and blood. Without any mention of the three terms. This is even taken to quotes after the eight given by Origen, Tertullian, Cyprian and Potamius. The claim is bumped up to around AD 400 and later, by Grantley Robert McDonald. (Recently I showed two of the later ones, Phoebadius of Agen and Victricius of Rouen.)
All this absurdity is one major reason why I see the current contra position as totally bankrupt scholastically.