I'm going to lock this as I'm not even sure if it was still an issue after the infant patch.
If you do still have issues with toddlers, please create a new thread.
I have the same issue. In my case it's related to the clubs system, specifically the combination of giving birth while being in a club meeting in a Club that has the Popular Club perk. After some testing I found the following:
Parents have a baby while not being in a club: normal relationship
Parents have a baby while being in a club, mother gives birth while in club meeting without Popular Club perk: normal relationship
Parents have a baby while being in a club, mother gives birth while in a club meeting with Popular Club perk: parents have reduced starting relationship, they need to introduce themselves to the toddler after aging up
Parents have a baby while being in a club, mother gives birth while not in a club meeting with Popular Club perk: normal relationship
So if your issue might be related to this, try ending club meetings before the mother gives birth (the relationship ended up normal even if labor started while in a club meeting, but the meeting was ended before the mother actually gave birth).
