I managed to see what you are talking about by doing some tests on my own end. The T-Pose that you are seeing by default is essentially a placeholder meant to represent the most common and default base pose for most skeletal rigs.
After you re-opened the Base Pose Editor and were seeing the T-Pose, try pressing View Pose again to see where you originally edited the base pose. This should be showing you the right position. Again, the T-Pose provides you with a visual reference as to the default position of your skeletal rig, so this is why when you don’t press view pose, you are seeing the T-Pose.