I have just had this problem. I've had no trouble with her face until just now.
I went into CAS to sort outfits out for my child who just grew up. She has 2 editable forms. Mum's a mermaid and her dad is Caleb and I think she is a hybrid because she has the mermaid icon on her needs tab but she is hissing in CAS. I had a bit of trouble with her editing, and she had an invisible face but I fixed that.
Her mum looked fine until I got out of CAS and she went swimming. She had an invisible face and her shell top turned into a suit jacket (I have no idea how). She does have a skin overlay but I had no trouble with it until now. I removed it though plus the 3d eyelashes but she still has the invisible face.
Edit: I think I found the problem. I have 2 swimsuits set for her. One of them is this one https://i.imgur.com/0MsEf0F.png If I put her in this and make her swim she loses her face and her top changes. https://i.imgur.com/d9NSWdt.png
If she wears the other one the bikini bottoms with the sheer wrap and the twisted top she is fine.
https://i.imgur.com/lzOVuw6.png