I believe this bug has been there for months, but anyway, it seems the detonation option for the Guardian’s Shield of Absorption sometimes fails to work with Charr characters, only.
Now, the worst thing about this is not only the fact that you cannot use it to do the AoE heal, but that it roots you on the ground until the whole effect fades, something quite serious if we talk about using it in sPvP, in which those few seconds lost could mean losing the fight as well.