Evocation Wizard’s Sculpt Spell Ability with spells that persist over time – under what conditions is protection maintained?

One of my current players has the first Wizard Evoker for whom I have GMed.

His PC’s Sculpt Spells ability was easy to interpret at lower levels, when his evocation spells, like thunderwave and fireball, were instantaneous.

As he advances, however, he may obtain spells that have longer durations, such as Storm Sphere, Wall of Ice, or Whirlwind. I am realizing that I do not know how to adjudicate Sculpt Spell with spells that persist, especially when the initial conditions of the Sculpt Spell ability no longer apply.

Sculpt Spells says:

Beginning at 2nd level, you can create pockets of relative safety within the effects of your evocation spells. When you cast an evocation spell that affects other creatures that you can see, you can choose a number of them equal to 1 + the spell’s level. The chosen creatures automatically succeed on their saving throws against the spell, and they take no damage if they would normally take half damage on a successful save.

Storm Sphere, for example, has a a duration of Concentration (up to 1 minute) and says in part:

Each creature in the sphere when it appears or that ends its turn there must succeed on a Strength saving throw or take 2d6 bludgeoning damage.

To use the Sculpt Spell ability, the Evoker must see the creatures he is designating as being protected at the time he casts the Storm Sphere, and they must be in the area of the sphere at the time he casts it. It is clear that designating the protected creatures happens "when you cast".

What is not clear to me is whether the protection is for just the first round ("when you cast") or is meant to persist and provide protection on subsequent rounds.

If the protection normally persists, can it be lost if the creatures no longer meet the conditions that were required for them to be designated as protected?

What would happen to a protected creature if it was to exit the area of the Sphere and later re-enter?

Does the Evoker need to maintain them in his sight to maintain their protection? If they were originally protected when the spell was cast, but later became invisible, would it end their protection if the Evoker could no longer see them?

It is possible that reviewers may mark this as asking multiple questions. To be clear, I see all of these as examples of one question – Under what conditions does the protection afforded by Sculpt Spell persist or not persist over time?