Unusable partition index

I've few questions related to global partition indexes.

1.For internal partition which are being added everyday do we need to update indexes for that.

2. We've a custom created package to drop few partition's as part of business needs. We're are dropping few partitions for ex:

   alter table <table_name> drop partition p1;

   alter table <table_name> drop partition p2;

   alter table <table_name> drop partition p3 update global indexes;

if we do it directly through commands our index will be in a valid state where as if we do it through package our index will be in unusable state.

I'm not able to understand why its behaving like that .