Does 'update global indexes' not work if the index is already unusable?
The update [global] indexes option does not rebuild indexes after whatever operation you ran. It maintains indexes during the operation (see Updating Indexes Automatically), so that they remain available throughout.

After your first truncate, the index needs to be rebuilt - it can't be updated, it's already "broken" and unavailable. Oracle won't rebuild it automatically for the next operation you do on the table, even if you include the update clause.