Just had this happen on PS4. I went to manage world's from live mode, moved a townie child to the household and then went back in to the active family. Their household collections completely reset. When I added the child to the household from the world selection/manage households directly after loading the game then load the active family the collections also get reset. I then moved the single child to a different household and then did the "ask to move in" interaction with the adult's household I moved the child to and the household collection did not reset.
Edit. PS4 Version 1.18
This happened upon moving to an apartment before Cats & Dogs was released. I think a new patch had gotten installed at the time as well.
