If I remember correctly there was an issue with collecting both plunder spells to begin with, which means when the issue was fixed, your plunder spells had different timers running, so one of them would've run out before the other.
I do not really pay attention to the time of the spells, but aren't they usually 7 days long? And the event ended more than 7 days ago, so the actual 'issue' is that one spell basically lasts two weeks, instead of one. (the inferno 5% spell in my case). Am i right?