I always hear this but I've never found this. The couple of times a year I eat McDonald's I check and it's the same. There are specific deals they'll have in the app like "2 filet of fish for $7 instead of $4 each" or something but by and large the prices are the same.
I don't know for sure but it may be regional. In my area, they always have a $5 off $20 deal that makes things slightly more palatable but even still, it's just bringing the prices back to what they really should be as others have said. I've uninstalled the app and completely stop going there entirely, the quality (not that it was ever great to be clear) is pretty terrible and there are so many better local places with way better value.