Miragai

A “dragon attack” is an attack (e.g. activated ability, attack action card) with the dragon sub-type. This does not apply to cards with “Dragon” in their name,

The effect only applies to the first dragon you control that attacks each turn. If you have attacked with a dragon, then play Miragi, the next dragon you attack would not lose phantasm.

Losing Phantasm this way does not prevent phantasm triggered-effects from resolving if the Phantasm has already triggered.