No relationship progress with babies

Hi! I've been having a weird error for a while now - since before the latest patch - that I only just got around to fiddling with. Whenever new babies are born in my Sims families, everyone in their household has 0 relationship progress with them. The relationship bars are at 0 and I have to have everyone introduce themselves to the newest member(s). Mods don't seem to have any effect - I've tried w/ all my mods, some of them, and next to none (w/ the only one in place being MCCC to speed up the pregnancy). The only thing that makes a difference is if it's a long-standing save file or not. I tried this w/ my pre-existing saves and no mod combination (or lack thereof) worked, but starting a brand-new save file with brand-new Sims did work. I even tried this w/ a family I hadn't played before in a pre-existing save, but that didn't work either. Does anyone have any ideas as to what's going on or how to fix it?