I hate it when they do that. Because what happens if you order a Strawberry Pie and they are out of Strawberry Pie? Then they'll offer you an Apple Pie as a less sexy substitution. lol.
McDonald's has had mobile ordering for 2 years and they still don't have their shit together.
It's rare that I ever order from a pizzeria online and they are ever out of something. Because pizzerias have perfected supply management since online ordering for pizza has been a thing for a very long time.
I make personal notes of which franchises tend to run out of stuff often whenever I make a mobile order. Because I hate playing the substitution or refund game. If you want a refund for a mobile order, you have to go through corporate I believe so I just take the subs. (I don't share these ****-ups with corporate since I don't want to be that guy. I know that franchises are having it rough and I imagine that employee morale at McDonald's locations in the GTA is very low being paid $14/hr when rent, auto insurance, health care, etc. is expensive. McDonald's employees tend to have better morale than Tim Horton's though at least)
The QR scanner at the register doesn't even always work too at some franchises with the app offers. They have major issues to work out with their app. Their app is very buggy too. I can be at a location with GPS enabled and it'll say "oops you're not there yet" at some places where the GPS signal may not be as strong. Like at a McDonald's in Wal-Mart. And lots of people have reported having their McDonald's Canada accounts hacked (either they are reusing passwords or have easy to bruteforce passwords). Their app has no bruteforce protection. No temporary lockout after numerous failed attempts.