Hey there,
Actually I have played through the story about a dozen times. The following situation happens everytime I reach the part when I can choose my mastery class: I am 3-4 levels above the unlock of the 3rd mastery skill. With most of the classes the mastery exclusive skill is one you aim for and thus you want to specialise in.
So as an experienced player you know that you will reach this story part soon and keep the 3rd specialisation slot free untill you get the mastery skill that you put in. But new players (as me on the first few playthroughs) will certainly choose a specialisation skill for the 3rd slot as soon as it is unlocked only to recognise they do not have a free slot when they get the mastery skill.
Unlocking the 4th slot is too far away, so you probably will despecialise the skill in the 3rd slot you just have leveled a bit. Also leaving the 3rd slot free to save it for the mastery skill feels like wasting xp.
So this feels out of place a bit.
Would be nice when the 3rd slot exactly gets unlocked with choosing the mastery. Or make it unlock at a later level that definitely lies after the common level people will get the mastery skill.
Kind Regards