Mesmer can spawn phantasms through doors.
did you actually target him to spawn the phantasms or target the gate and he got hit by the AoE?
Phantasms often tend to spawn behind the target. So since the Mesmer stands in front of the door (and target’s it), his phantasms often spawn on the other side (and target the door).
But heck, why do you complain as enemy ? I mean, it’s free kill of his phantasms, while he/they can’t harm you.
Being a mesmer, it really sucks when that happens. Phantasms grant regeneration, retaliation, reflection and damage reduction, why the hell whould I prefer my phantasm to spawn on the enemies’ side?
As for spawning phantasms outside of LoS:
this has been fixed quite a while ago. Now it’s aweful since all mesmers skill go on cooldown, while doing nothing. And the can be blocked thrice. As easily as you can block rangers arrow rain by dodging once, blinding him, going behind an obstacle or killing the skill itself (sarcasm).