Been reporting this since the headstart, and haven’t heard anything or seen a resolution for this problem.
When casting some mesmer clone skills that cause a short teleport – specifically, Phase Retreat, though I have had is happen with Mirror Images as well – the caster will sometimes teleport under to floor. This usually occurs in areas that have terrain seams with objects underneath you, such as in underground area, or indoors, but it also happens regular when around rivers/streams or any hilly/mountainous area.
This wouldn’t be quite so much of an issue if we had some way to resolve this besides utilizing a waypoint – forcing the player to spend money that they shouldn’t have to. However, the areas that under the terrain floor are often filled with water, so we are swapped to our underwater weapon sets, which have no skill similar to Phase Retreat. Blink is also disabled underwater.
The one forum post that got a response said it would be handled on a “case-by-case basis,” but I have experienced this so many times in so many areas, that I believe it to be more of a global issue with these skills. I have also seen a post regarding the apparent use of this in WvW, which was not responded to.
I’m attaching a screenshot of the latest occurrence of this, after swimming around in the vast ocean that is apparently under the Iron Marches zone. I could not get the exact position it occurred in due to mobs following me under the zone and continuing to fight.

