It's a murky world, but I think that when selecting a currency other than the native one, you are simply subject to TK's internal exchange rates. The underlying ticket price remains denominated in, say, lira (TL). When a refund is made you receive the same number of TL, but converted to your selected currency at that day's exchange rate. With the lira on an inexorable slide that means you can be pretty certain you'll get back fewer euros than you paid.

You night want to explore whether or not your credit card folk believe that is a fair way of doing business. After all, from their point of view it was a transaction conducted solely in euros.