The developers reverted back to 2.522294, as it turned out that currently 2.6 versions don’t work properly with the so-called “bot-play” in the “Lobby”.
This does not affect play by forum of course.
In case you play lobby/bot you should use 2.5.22294 to do so.
People who prefer to use a current pre-release can download those here:
https://github.com/triplea-game/triplea/releases/