Hi John,
There are a couple of places you can check for filters. The first being the activity status filter on the left hand side of the calendar, verify if any of the filters have been checked. The second place is the calendar preferences step 7 of 7, there should only be 3 filters; if the activity is a work order, the status is not 'cancel' or 'waiting' then they should show up on the calendar. If you have anything different then that may be the reason why the work order disappears after it has been invoiced. I tested this on my end and it works fine.