Having the same issue. Started with my 4th infant of this family, who could not gain the 'sit up' milestone. It would say she had got it, the family would gather, but no pop-up or unlock would occur. This interaction would spam repeatedly every time I put her in the playmat, to the point where I had to give up and delete the playmat entirely until she aged up.
I'm on my 5th infant now, no spamming is occurring, but she can't seem to gain milestones at all. She got the 'born' and 'smile', but beyond that, she's just not achieving anything.
So confusing, because I had no issues with the past 3 infants at all.