My firm has had this same issue. It is completely random, and has happened to multiple users in various studios.
The x-base is inserted into the sheet file and all is good. Notes, dimensions etc are added, and away we go. Then sometimes, without rhyme or reason, you will go into sheet, and the base is missing. Switching to model space shows that they entire base plan has moved. Most recently, it was only about 14". Other times it has been hundreds of feet. Everything checks out in the sheet file, so you go to the x-base and find the issue. Somehow, when you run the base command, the basepoint of the drawing has been changed.
This has happened to me a few times now. Often I was the only one in the base. I had closed all files the day before, and it was fine. Upon coming in the next day, this had happened. Nothing was changed manually. An Audit shows no issues. changing the base back to 0,0,0 fixes the issue, but it is frustrating when you are trying to get a set of drawings out and have to stop to fix things that should have been set.
Several people in our firm have researched this, and found no other solutions or possible causes. Currently using AutoCAD 2022 LT, but it was doing it with 2020 LT also. If it is a bug, it has been around for a long time.