I have a recurring event every week at 15:00, but I changed the event today (as an exception) to start at 16:15. However when the reminder triggered 15 minutes before at 16:00 (which is OK), the dialog popping up still shows Time: 15:00 - 15:45
Confirmed with 5.17.2. The reminder dialog's "Date Time" column and the details pane both show the base event's time.
Re-confirmed for kalendarac. The notification that pops up says "Event starts at «base start time»", not "Event starts at «exception start time»", and when it counts down ("Event starts in «n» minutes") it counts down to the base start time.