What's the map name you are engaging the mob at? (In-game type /where to get the exact location).
Could be a drain SP skill, but I don't recall said mob has it.
Today has happened to me at iz_dun02 , but I have experienced this bug at gef_fild01 too a few months ago...
The mob doesn't have any SP drain but it is related to this TharaFrog somehow, since has only happened to me while fighting it. Moreover it just looks like the SP bar gets bugged since the % value disappears for a moment.
Edit: Sadly I don't know how to trigger it, but I will try to be ready the next time it happens to me and take a screenshot...
Cheers,
Edited by saludd, 20 March 2018 - 05:38 AM.