It’s perfectly normal to see a difference in the time line vs the preview monitor. Your preview is capable of viewing the video at a frame by frame basis at any time. Meanwhile… your time line may or may not have this capacity depending on what zoom level you have it set at. If you have it zoomed all the way out for example then it may be showing something 10,20…. 100 frames away from what is on the monitor.
As for the shift… if you accidentally change the zoom level on the time line at the wrong time….
This has happened to me quite a few times as I accidentally bump the zoom wheel on the mouse sometimes.