Looks like this has been patched now.
The idea that it's caused by the interaction with Mind Buff (upgraded) and Barrier seems right - it does trigger when you barrier yourself and then hit an enemy with Mind Blast. At least, I've been able to reproduce the effect multiple times by doing so. It's strange that it doesn't create any status icon with it, and you might be right that the visual isn't supposed to persist after combat. Would be interesting to know if it's working as intended? But there are so many other threads going on that I don't know if we'd get an answer.
Took some screenshots of it myself, for what it's worth:
http://i.imgur.com/hy5fROq.png
http://i.imgur.com/Ln4VVir.png
http://i.imgur.com/5mHcBa1.png
Good point, I don't know if it's actually functioning as intended. A lot of mage abilities appear to be bugged, from what I've seen around the forums - this might just be another (minor) issue. And yeah, the missing status icon is a bit suspicious.
Maybe when/if the mage abilities get patched, this will get resolved too? There's so many problems with the game right now, including Solas and Dorian's quest lines, that a little visual glitch like this probably isn't a priority.
