Lightable shows the thumbnail preview, if it exists in Cache it shows that version, if not it generates it. Darktable will show you the actual image plus edits, when you finish edits the thumbnail preview is regenerated. So I "guess" that you are looking at previews generated under the older version and when you go into darktable it shows the actual image and edits. I have not used 3 much yet so I have not noticed any difference, but I did notice that there seemed a lot more modules in the history stack so "maybe - another guess" version 3 is loading more modules as defaults. So maybe looking at the history stack. I also "heard, so d'no if true" that the module pipe that was fixed in earlier versions has been amended so "maybe" the modules are now in a different sequence. These details should be on the darktable website
Steve