I was playing aram today and have never played Zyra before and thee was a sivir on the enemy team. I spent the first 12 minutes doing everything possible not to pop sivir's shield when around that time I noticed that she was gaining mana from the thornspitter's autoattacks. I didn't think those counted as spellcasts and it really hurt our team with me basically giving sivir free mana regen.

Is this a bug or is it working as intended?

its damage from a spell and its not a pool effect spell so should affect spellshields, but then again sivers spell shield is different from other spellshields

no.. the shield acts to block any ability attack.. so yer plants count as an ability...