I faced the same issue with a customer's S20+. Display-assembly replacement came with small-parts, including a front-facing camera. After installing everything into the replacement display, everything worked except the front-facing camera. So I took it apart again and swapped in the old front-facing camera. Just takes some heat and careful prying. I removed the epoxy first, using curved tweezers, and saved it. The camera popped right out by gently prying it with a plastic spudger. Installed the original camera into the replacement display-assembly, and stuffed the epoxy back in. Everything worked out.