You shouldn't be having to pay for any drinks at all whenever you're working (even if it's like right after your shift). You only pay for food (with the discount) if you have more than just your one free food item per day.

The normal benefit is free drinks while working, one before and after your shift, and one free food item per shift, and 30% off the rest of the time.

Since April 2020 there's been a temporary benefit, currently extended through June, where you can also get one free drink and food item on days that you don't work. That's probably what you've been getting. It's not automatic or anything, so the barista ringing you up is using the markout button to apply that benefit instead of the discount button.

hey fam! welcome. on ur days off, you’re entitled to one free food and one free beverage. this means that when you go to a starbucks outside ur home store, the register barista may assume that its ur day off and mark out your drink/food for free. also the 30% off is not for the whole order, but each individual item gets 30% off.