Guild Wars 2

The additional charge from mesmers’ Harmonious Mantras trait is lost each time the character’s level gets readjusted.
For example, if a level 80 mesmer runs through the Diessa Plateau and crosses the border between an area where he is level 24 to an area where he is level 26 (or vice versa) and has one or more readied mantras with 3 charges they each lose the 3rd charge.

Just to bump this with more info, it also seems to be happening if the mesmer is downed.