Generations gone

So, I had a legacy challenge and I was on about the 5th generation. I backed up my PC recently, it messed with some of my mods but I reinstalled them. I went back on, most of my saves were fine other than a few of the ones I had already deleted coming back for some reason. The rest were fine as I left them, except for my legacy save. It had reset back to my starter couple (first generation) just after they had their first kid. I don’t know why this happened, this was the only non-deleted save that was affected and the rest were completely fine. Four generations were lost and I don’t know why.