1.5k ticks are nearly impossible in PvP.
The cause for what you see in the screenshot is lag. I’ve had 2k terror ticks because of lag earlier, and it’s the only possible explanation.
@OP: Can you confirm having lag anytime during that fight? The missed condi dmage would be added to the next condi tick, so it would be around ~750 dmg ticks without lag, and that’s quite reasonable.