Why is this even a thing? It s beyond me.
It's a thing because, for some reason, they decided to make it so that when a skill is not currently available, it is removed from your skillbook and action bars.
I don't get why they did such a thing, but they did and it's why the action bars in BG3 suck so bad, especially things like temporary actions and Wildshapes.
As far as Warlock and Hex go... There is a particular action bar that exists where temporary actions like Continuous Hex naturally appear which can alleviate the need to open the skill book and drag it down.
Though, I personally ended up just ditching Hex because of this annoyance with Continuous Hex (As well as the need to always pick a stat to debuff like any of them actually matter when only one of them can do anything and even then only in 0.01% of cases if you're doing some very specific action). Instead I erred towards alternate spells like Cloud of Daggers, Hunger of Hadar and Wall of Fire to spend concentration on.