This is happening to me as well, to where I don't join them at the Hospital when they have a baby. Recently, I had a sim in the Doctor career. I let her have a baby at the hospital and joined her. Now when she goes to work, she changes into a hospital gown instead of her Doctor coat.
I just started having this issue. I usually have my sims have the baby at home so it hasn't come up before but I wanted the birth certificates for this save. Looks like this has been a problem for at least a year and half, is anyone working on it?