Here are a few things to note about the expiration date for proposals.
The number of days the proposal is given before it expires is set in your account settings under "Order Settings". Your's is set for 60 days.
This date is set on the proposals on the day that you create the proposal. So if you created 3 proposals today 8/9/17 all 3 would have an expiration date of 10/8/17. The idea being that when a proposal is created it's then sent off to your client.
If the event is sooner than 60 days, say 9/15/17, then that is how the expiration date could be after the event date. It's based on the proposals creation date not the event date.
Where this differs is with Wishlist orders. Because these aren't actual proposals yet they do not have an expiration date and so the field is blank. Once the wishlist has it's order status changed to Proposal it will set the expiration date for 60 days from that time.