Idk if yall figured out the solution on your own and I know this is an old thread but I had the same issue and just solved it myself. When you go to Settings in the calendar app, under the section Reminder Settings, you need to select the box next to "All Calendars". And then reselect the Default Reminder Time and it's done.
*I set this on the first HTC one I had and it defaulted but when I had to get it replaced by warranty, the calendar app wasn't defaulting my choice like the first phone and. I just really this. Because originally the phone was only setting your reminder for whatever calendar account you selected. When you select all Calendars that calendar choice disappears.