I have no way to prove this but I'm just going to say what happened.
1. Created small deep biome
2. Explored it
3. Found 2 elder apostate...
4. One of them exploded
5. I mined the other one
-it killed me and said I got an elder apostate...
6. I have no elder apostate???
7. I read the word "Elder" on my screen but check and no elder anywhere