When using "Shadow Meld" (2 sec stealth) and being marked. Shadow Meld behave like rolling a dice... sometime it reveal you other time it don't as shown in video below? (Maybe make marked OR stealthing > 2.1 sec or < 1.9 as the cut off to avoid this in-between behavior).
As you see in video, I casted shadow meld twice, first time it reveal me, 2nd time it did NOT while being marked from a sentry.
In other words, depending on your luck when marked, those 2 sec will either reveal you or NOT reveal you.
As it was, shadow Meld did NOT have this issue in WvW as you never use it until you get revealed. Now that it is a stealth only skill, I believe this should be kindly addressed/adjusted to make it viable/functional when fighting near Watchtowers or Sentry in WvW and fix this "DICE" rolling behavior.