Honestly I'm surprised this was noticed at all... I did in fact change this behavior in 2.6. I added a feature in beta 4 to auto-perform an action (such as buy-in) when a search returns only a single result. So for you barcode reader users, if the buy-in dialog is open and and this feature is enabled, when you scan a barcode it will automatically buy that player in (if a match is found) without the need for additional keystrokes. In working on this, I got to work out the find feature a little more and felt the way it worked was a little unintuitive, and wrong frankly. The dialog is "smart" in that it remembers which players are checked and which are not, even if the list of players is updated by using the Find button. So, for example, if you are buying players in and check several, then are having trouble finding a specific player and use the Find feature, it would be annoying after finding and checking that last player to return to the full player list (by pressing Clear) only to find all your checked players unchecked. So the dialog remembers who is and is not checked to avoid this. However, previously when you pressed OK, every player that was checked would be bought-in, regardless of whether or not that player was currently visible in the dialog (so if you check 10 players, then press Find and get no results - thus no players currently listed - then press OK, the 10 players are bought-in). I changed this so that only currently visible checked players are bought-in. So in the previous example, no players would be bought-in when OK is pressed because no players would be visible.
I think one could make a convincing argument for the previous behavior, but I'm leaning pretty hard towards the new way. But I'm definitely interested to hear opinions.