Sivir/Gangplank ult bug???

I was playing Sivir the other day and during mid game I was caught under a GP ult now I hit my Spell Shield right as the ult began on top of me dead center and it did proc. that however wasn't the odd part what was odd is that once my Shield proc'd I was No longer taking damage or slow from thw still going ult. now I'm not sure if this is a bug or not because my confusion is with the GP ult classification.. I assumed that GP ult worked on a tick bases that resets it's self until the ult is done so with that being said my Shield would only block the first tick of dmg and cc yet after it would reset and I would start taking dmg but if that is true her Shield should never of proc'd because it doesn't work on moves like that. I also could of just missed judged what i show and was taking dmg and something else could of proc'd my shield. but only other thing around me was caster minions and they don't count.....I will keep a eye on this and see if I can repeat this effect and any input from the community would be greatly appreciated