OK. It does the same thing in the sandbox. I added two Thursday weekly events. One starts at 2000, with a reminder to go out 6 hours before the event. Today's event says a reminder will go out at 1401. The future events all show the same one-week early reminder as my team's calendar. Here are the steps I took to initially create the event:
In Calendar, I hit "Add an Event".
When the new event came up, I filled in the event information.
I then went to the tab where I selected the invitees.
I did not require an RSVP.
I set it to be a repeating event, every week for 100 weeks.
I then selected one reminder to go out six hours before the event.
I then saved the event.
I then created a second event the same way, but at 2030, with a reminder to go out 6 hours prior to the event. It shows the reminder will also go out at 1401, not 1430 or 1431. I then went to next week's 2030 Thursday event. I turned off the reminder (to clear it) then turned it back on with a 6 hour setting. I then updated the event and series. It made next week's 2030 reminder correct, but changed all the others just like my team's.
It doesn't seem to like Thursdays.