Its not a state based effect. Its a triggered ability that triggers on a player casting a spell. It should have triggered when lotushead cast sphere, before he passes priority.
To me the question is whether it is ok to not announce a trigger that happens until significantly later in order to gain an advantage.
You don't have to announce triggers when they trigger, only by the time they would produce observable effects. The classic example is exalted...swing with a guy 2/2, "no blocks". "take 3" is acceptable; whereas that sentence with "take 2" is a missed trigger. [note this ipg example is exactly opposite to an ipg example used before the last change, which is part of what confuses people]
MTR:
4.4
Triggered Abilities
Players are expected to remember their own triggered abilities; intentionally ignoring one is Cheating.
Players are not required to point out the existence of triggered abilities that they do not control, though they may do so within a turn if they wish.
Triggered abilities are considered to be forgotten by their controller once they have taken an action past the point where the triggered ability would have an
observable impact on the game. Triggered abilities that are forgotten are not considered to have gone onto the stack.
The IPG section 2.1 also makes this clear:
2.1.
Game Play Error—Missed Trigger
Definition
A triggered ability triggers, but the player controlling the ability doesn’t demonstrate awareness of the trigger’s existence the first time that it would affect the game
in a visible fashion.