I'm not saying it makes any sense. I'm trying to approach it with how it is likely coded atm. We really need to try more shields like Lulu's to see if it works the same as janna's or BT. Then we can diagnose if this is an intended result or a error that needs to be fixed. If they want it to work so that all item shields work one way and all champ shields work another than thats "ok" because at least there is some consistency, but if some items shields work one way and some champ shields work that same way, but not all then we really have a problem.
Either way I think her passive needs an update. I saw a thread that talked about making it like Garen's passive where it isn't affected by small minions after lvl 11 for a QoL change.