I had the same thing happen to me two weeks ago as @deevecchione: We had a 10:30 FP for Soarin (booked ~60 days in advance) which we decided to change to Test Track on the day before the FPP was scheduled for. (Soarin’s line even at 9:30 went pretty quick the day before, so we decided we’d rather ride Soarin SB, and FPP TT).
On the app I chose “change attraction” and it showed Test Track as available, and it still showed the 10:30AM time at the top of the screen. I hit Confirm, only to go back to my reservations screen to see it gave me a 4PM Test Track! Nowhere before I clicked “Confirm” did it indicate it was changing the time! Fortunately this time worked for us that day b/c we had a 5PM ADR in EP, but most other days it would have been a lost Tier 1 FPP b/c we wouldn’t have been in the same park for the evening.
Same thing happened a day or two later on a MK day… tried changing a morning attraction FPP to Peter Pan, and while it showed the same time frame on the “Confirm” screen, it booked PP MUCH later in the day.
Hopefully they eventually update the app to show the new attraction’s time BEFORE you confirm it.