Does a bitcoin exchange transfers my bitcoins to a "temporary address" when an order is placed (and not yet executed)?
I don't have any specific knowledge about Bitfinex, but I doubt that it works that way.

The most straighforward way to run an exchange is to pool all customer deposits.  When you make a deposit (to a "t-addr"), the exchange credits your account (in their internal books), and the coins in that address are considered part of the "pool" and no longer have anything to do with you specifically.  They're not "escrowed" in any way; they literally belong to the exchange now, and you have to trust that the corresponding credit in your account will be paid out by the exchange upon your demand.  And there's no need to move coins gratuitously to avoid double spending; it wouldn't help.

Any external transfers that have to be made from the pool (customer withdrawals, moves to cold storage, Bitfinex extracting their earned fees, etc) are made from arbitrarily selected pool addresses.  Trades that take place on the exchange can be done just by crediting and debiting the internal accounts of the customers involved, and typically would not provoke any external transactions, neither in cryptocurrencies (on the blockchains) nor in fiat (in actual bank accounts), until one of the customers wants to make a withdrawal.  That would just be a needless expense in (external) transaction fees.

I suspect the blockchain transaction you saw around the time of your trade was just a coincidence, meaning only that the exchange needed to move funds for some reason (as described above) and happened to choose "your" address for that purpose.