AT LEAST since the last Episode before Edge of Fate, after switching from using a glaive to another equipped weapon sometimes prevents use of a fully charged Powered Melee ability. Sometimes doing a bunch of other things in the activity might eventually let it be used, but it's rare. It's almost exclusively resolved on death and respawn. Now, it seems to also sometimes happen when switching from using a sword, yet not as frequently as a glaive, and even just at random times, however that's much less frequently than the other two situations. I'm primarily playing as a Titan, using Prismatic, with the Void Shield Throw Powered Melee. I don't think there's any visual shown on screen when trying to use the ability, such as the icon flashing red when trying to use one that's not fully charged. It's not limited to just one piece of gear or slot, since they all change.