The "Expire End of Month" relates to limit orders - in HSX each trade is atomic and doesn't relate to any others. This is probably easier with examples:
If you say you want to short 50000 shares of AAAAA, then Expire End of Month will be being displayed, but it has no effect unless you enter a Limit Price, the trade will go though there and then.
If you enter a Limit Price (e.g. 50) then you are saying that you want to short 50000 shares of AAAAA when the price reaches 50, as long as it reaches that price by the end of the month. What happens then will depend in the price of AAAAA.
If AAAAA is priced at 45 then the order will go in a waiting area (Limit Orders) until the price of AAAAA reaches or exceeds 50, in which case the short will be executed, or it comes to the end of the month, in which case the order will be expired.
If AAAAA is priced at 50 or over then the trade will be executed straight away as the limit condition has already been met.
This is the same for all types of properties - movie stocks, star bonds, etc. - it only appeared to work for the movie stock because the movie stock delists off the exchange automatically after 4 weekends of being wide released - it was nothing to do with "Expire End of Month" when you made the short.