This problem happens when I am not looking, but here is a guess:
From time to time a recently used memorized transaction will disappear from all three of the places that I know about to look for them(List Manager, Calendar view and the star icon). Later, I will notice that some other memorized transaction name now contains the missing transaction data instead of the data that it is supposed to have. It seems to even have the wrong next entry date. In fact, that is how I found the most recent incorrect transaction.
In today's case, I think that the memorized transaction that became corrupted was used earlier in the same data entry session.
Since a transaction is lost, I have started renaming the corrupted transaction as bad so I can remember what needs to be re-created. This is happening in a working file with exactly 70 memorized transactions. I do not know whether I can reproduce it in a small file.
Happy Hunting.
Dave