Possible Sejauni Frost Armor bug

So, I was playing ADC and had an Illaoi support while the enemy team had a Sejuani support. I noticed something with Illaoi's Test of Spirit while Sej's passive is active. Typically, you break the Frost Armor and remove her resistances but when Illaoi pulls Sej's spirit while Frost Armor is active, the spirit will have the bonus resistances no matter how many times you attack it. I could understand if you could hit it and it would be like normal Sej passive where it would break and remain for a few seconds, but the bonus armor and MR stayed completely. Maybe this is just a built in mechanic, but I'm not too sure.