Koha will allow receipt of an issue on an expired subscription. Bug 29188 already exists to suggest we should no longer allow that. I'd be fine with preventing receipt of issues on expired subscriptions entirely, but for the moment Koha allows it. In 20.05 and earlier, Koha would correctly follow the serialsadditems when receiving an issue on an expired subscription. In 21.05, Koha will not create an item when receiving an item on an expired subscription, even if serialsadditems=1. It allows the receipt, but does not present the Add Item form. If we're not preventing this receipt entirely, we should make it work entirely.