Completely non-scientific observation, but it seems to me like the issues of invisible enemies has gotten worse in the last few days. Attacking a tower or keep, it seems to be the norm now that players will not appear on the battlements, but projectiles will be visible coming from where the players should be (but aren’t being) rendered by the game client. Similar issues seem to happen more frequently in the open field lately as well.
Now, some people claim that “smart” groups are taking advantage of mesmer/thief group invisibility skills, rotating them to provide near 100% uptime, also utilizing combo fields for stealth/invisibility effects to prevent themselves from being rendered. I can’t say with 100% certainty that I’ve seen this behavior, but it seems like someone (or many people) have figured out that if they use stealth fields, invisibility bubbles, etc…that they won’t show back up on their opponents’ game clients until well after the effect should have ended…and in some instances, they never re-appear at all…