I would imagine the reason they are now up in the daytime is that in MF (town in SN) that the ghosts are up in the daytime, not restricted to just night time there. Ghosts can be fun, but also a royal pain.
If you want to keep them around, but not have them be a pest, put them in one of your sims inventory. That way, if or when you move you will be taking them with you. You can also control how often you want them to come out. In a project family I am playing I set out the tombstones for the founders and have let them come out for my story. As soon as the daughter-in-law dies, I will put the founders and their son with her tombstones back into the oldest inventory. I want to keep them with the town jumps but not being a pest every night.