I have only found closed instances of this issue. Needs reopened.
Mermaid in question is Paka'a Uha. I haven't edited him in any way in this save that has only seen a mermaid in Sulani the very first time. Pics attached.
Mermaid tail is invisible on sim in CAS as well. Switching between forms don't seem to refresh state of tail. After applying tail in CAS tail looks fine, then disappears as entering back to living mode.
I don't know if it's worth mentioning previously the issue was a young adult trait? Also I don't know if this is CAS, or graphics related.
