The thing happened to me too while soloing him a few days ago, only that time he died from "Rasp II" while I was stunned. And when it happened to Jonas I was in his Party.

This seems to always happen with conditional status effects, like Stun, Bind, etc. which either wear off over time or with the next attack of the inflictor. What happened to Jonas was the following: The Prince of Pestilence casted Shock Spikes on himself, Jonas hit it and got the Stun status affect inflicted on him. While Jonas was still stunned the Prince was killed by a party member which bugged the debuff out.

Platform: Windows Vista x64 Ultimate
ISP: Alice (german)
Type of Internet Connection: DSL
Internet Connection Speed: 16Mbit/s
Date & Time: unknown
Frequency: Sometimes
Character Name: Rhya Neema
Race: Miqote F
World: Wutai
Class/Skill Rank:
Area and Coordinates: The Mun Tuy Cellars, 6-4
Party or Solo: Party and Solo
NPC Name: none
Monster Name: Prince of Pestilence
In-Game Time: unknown

Steps:

1. Start the game
2. Attack the Prince of Pestilence
3. Drain the Prince of Pestilences health until it is low
4. Wait until he casts Shock Spikes on himself
5. Hit him and get the Stun status effect inflicted on yourself
6. Have a party member kill the Prince of Pestilence shortly after the Stun was inflicted and while it is still active.

Results:

The timer on the Stun status effect will reach 0:00 but will never wear off, as will all the other buffs currently on the player.

Expected Results:

The status effect should be removed once the timer reached 0:00 and not affect other buffs.