Sims forget already existing relationships

On PC, never had any mods in this save, all my packs disabled except Get to Work and most kits.

Every so often the members of my family will have their relationship progress culled, sometimes even mid-conversation. This result in parents suddenly not knowing their toddlers/children/teens and siblings forgetting about each other despite living in the same household and interacting frequently. It's quite annoying to have a child with maxed out friendship with their father bragging about her grades one second and then having to make friendly introductions the next. They still seems to keep their family tree relationships though, so they are still father and daughter etc, they just act like they never met. 

So far I've caught it happening 4-5 times in the same save/household but between different sims and at different life stages. If it makes a difference the household consists a human mother, an alien father, their 3 half-alien kids and another full alien (step/)daughter because the father got abducted on their wedding night. The mother has never been involved in the culled relationships to this point, but everyone else has.