Spells: DOTs refreshing - or not

[Dangermouse]

I'm not certain about this yet, but I thought I'd make a post in case anyone else has seen it.

I was bashing the bard (ooer) in the Feerrott, with my shaman. I usually stick an affliction at the start of the fight then follow with envenomed breath until she's dead. Affliction (disease DOT) lasts almost the entire fight. But this time I accidentally cast affliction again instead of EB. Then a couple of ticks later got "Your affliction has worn off".

I would have expected recasting a DOT on a target to restart that DOT's timer - in this case 21 ticks. I checked and the affliction did land, it wasn't resisted.

I've only got one data point, but I thought I'd check if this was "working as intended" or a potential bug before investigating further.