This problem has come up several times in the past few years. There are multiple threads about it. I am currently experiencing it, too, but irregularly. I don’t remember that anyone has ever pinpointed the problem. Experiments by @smurph indicated that it was not due to the length of the session, nor the number of variations examined. Here is one of the old threads: Analyze Game Sessions Disappearing During Use
Found it. Here is the older, richer discussion: Game analysis randomly resets